Resolve four broken refs in the docs

Hello!

## Pull Request overview
* Resolve 4 broken references in the docs

## The problems
Two broken references [here](https://huggingface.co/docs/datasets/package_reference/main_classes#datasets.Dataset.class_encode_column): 
![image](https://user-images.githubusercontent.com/37621491/220056232-366b64dc-33c9-461b-8f82-1ac4aa570280.png)

---

One broken reference [here](https://huggingface.co/docs/datasets/package_reference/main_classes#datasets.Dataset.unique): 
![image](https://user-images.githubusercontent.com/37621491/220057135-2f249d60-c01d-48b5-82bb-5085a7635198.png)

---

One missing reference [here](https://huggingface.co/docs/datasets/v2.9.0/en/package_reference/main_classes#datasets.DatasetDict.class_encode_column):
![image](https://user-images.githubusercontent.com/37621491/220057025-4a8e5556-5041-4ec7-b8d8-ed4fdc266495.png)

- Tom Aarsen

Comments:
_The documentation is not available anymore as the PR was closed or merged._

See the resolved changes [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5550/en/package_reference/main_classes#datasets.Dataset.class_encode_column), [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5550/en/package_reference/main_classes#datasets.Dataset.unique) and [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5550/en/package_reference/main_classes#datasets.DatasetDict.class_encode_column), respectively

<details>
<summary>Show benchmarks</summary>

PyArrow==6.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.008256 / 0.011353 (-0.003097) | 0.004400 / 0.011008 (-0.006608) | 0.098676 / 0.038508 (0.060168) | 0.028937 / 0.023109 (0.005828) | 0.302578 / 0.275898 (0.026680) | 0.334170 / 0.323480 (0.010690) | 0.006657 / 0.007986 (-0.001329) | 0.004581 / 0.004328 (0.000253) | 0.076874 / 0.004250 (0.072624) | 0.034401 / 0.037052 (-0.002652) | 0.303928 / 0.258489 (0.045439) | 0.348421 / 0.293841 (0.054580) | 0.033303 / 0.128546 (-0.095243) | 0.011445 / 0.075646 (-0.064202) | 0.322137 / 0.419271 (-0.097135) | 0.041072 / 0.043533 (-0.002461) | 0.306007 / 0.255139 (0.050868) | 0.325945 / 0.283200 (0.042745) | 0.086685 / 0.141683 (-0.054998) | 1.454956 / 1.452155 (0.002801) | 1.545525 / 1.492716 (0.052809) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.175536 / 0.018006 (0.157530) | 0.400203 / 0.000490 (0.399713) | 0.002103 / 0.000200 (0.001903) | 0.000072 / 0.000054 (0.000018) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022750 / 0.037411 (-0.014661) | 0.095163 / 0.014526 (0.080637) | 0.103995 / 0.176557 (-0.072561) | 0.138806 / 0.737135 (-0.598330) | 0.105711 / 0.296338 (-0.190628) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.427860 / 0.215209 (0.212651) | 4.259594 / 2.077655 (2.181940) | 2.157986 / 1.504120 (0.653866) | 1.913814 / 1.541195 (0.372619) | 1.793455 / 1.468490 (0.324965) | 0.702341 / 4.584777 (-3.882436) | 3.353086 / 3.745712 (-0.392626) | 1.856952 / 5.269862 (-3.412909) | 1.149963 / 4.565676 (-3.415713) | 0.082926 / 0.424275 (-0.341349) | 0.012307 / 0.007607 (0.004700) | 0.524531 / 0.226044 (0.298487) | 5.254766 / 2.268929 (2.985838) | 2.590157 / 55.444624 (-52.854468) | 2.272613 / 6.876477 (-4.603864) | 2.304367 / 2.142072 (0.162294) | 0.819298 / 4.805227 (-3.985929) | 0.152170 / 6.500664 (-6.348494) | 0.066563 / 0.075469 (-0.008906) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.205054 / 1.841788 (-0.636733) | 13.729073 / 8.074308 (5.654765) | 14.061037 / 10.191392 (3.869645) | 0.138020 / 0.680424 (-0.542404) | 0.028042 / 0.534201 (-0.506159) | 0.392260 / 0.579283 (-0.187024) | 0.405632 / 0.434364 (-0.028732) | 0.469583 / 0.540337 (-0.070755) | 0.563110 / 1.386936 (-0.823826) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.006513 / 0.011353 (-0.004839) | 0.004402 / 0.011008 (-0.006606) | 0.076339 / 0.038508 (0.037831) | 0.027222 / 0.023109 (0.004112) | 0.338968 / 0.275898 (0.063070) | 0.378475 / 0.323480 (0.054995) | 0.005443 / 0.007986 (-0.002542) | 0.003312 / 0.004328 (-0.001016) | 0.075352 / 0.004250 (0.071102) | 0.034951 / 0.037052 (-0.002102) | 0.342268 / 0.258489 (0.083779) | 0.381024 / 0.293841 (0.087183) | 0.031568 / 0.128546 (-0.096979) | 0.011558 / 0.075646 (-0.064088) | 0.085267 / 0.419271 (-0.334005) | 0.041248 / 0.043533 (-0.002284) | 0.340422 / 0.255139 (0.085283) | 0.365497 / 0.283200 (0.082297) | 0.088278 / 0.141683 (-0.053405) | 1.479838 / 1.452155 (0.027683) | 1.554440 / 1.492716 (0.061724) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.223240 / 0.018006 (0.205234) | 0.394771 / 0.000490 (0.394282) | 0.003022 / 0.000200 (0.002822) | 0.000071 / 0.000054 (0.000016) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.024842 / 0.037411 (-0.012570) | 0.099167 / 0.014526 (0.084641) | 0.106376 / 0.176557 (-0.070180) | 0.141397 / 0.737135 (-0.595738) | 0.110355 / 0.296338 (-0.185983) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.437598 / 0.215209 (0.222389) | 4.394964 / 2.077655 (2.317310) | 2.082660 / 1.504120 (0.578540) | 1.868690 / 1.541195 (0.327496) | 1.915190 / 1.468490 (0.446700) | 0.701035 / 4.584777 (-3.883742) | 3.306594 / 3.745712 (-0.439118) | 1.842681 / 5.269862 (-3.427181) | 1.155022 / 4.565676 (-3.410654) | 0.083310 / 0.424275 (-0.340965) | 0.012413 / 0.007607 (0.004806) | 0.543179 / 0.226044 (0.317135) | 5.445605 / 2.268929 (3.176676) | 2.545080 / 55.444624 (-52.899544) | 2.188741 / 6.876477 (-4.687736) | 2.205561 / 2.142072 (0.063489) | 0.804967 / 4.805227 (-4.000261) | 0.151024 / 6.500664 (-6.349640) | 0.066448 / 0.075469 (-0.009021) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.304671 / 1.841788 (-0.537117) | 13.996631 / 8.074308 (5.922323) | 13.617626 / 10.191392 (3.426234) | 0.141512 / 0.680424 (-0.538912) | 0.016527 / 0.534201 (-0.517674) | 0.384981 / 0.579283 (-0.194302) | 0.385198 / 0.434364 (-0.049166) | 0.469033 / 0.540337 (-0.071305) | 0.554738 / 1.386936 (-0.832198) |

</details>
</details>

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