Misc doc improvements

Removes the warning about requiring to write a dataset loading script to define multiple configurations, as the README YAML can be used instead (for simple cases). Also, deletes the section about using the `BatchSampler` in `torch<=1.12.1` to speed up loading, as `torch 1.12.1` is over a year old (and `torch 2.0` has been out for a while).

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

PyArrow==8.0.0

<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) | 0.006616 / 0.011353 (-0.004737) | 0.003915 / 0.011008 (-0.007093) | 0.083271 / 0.038508 (0.044763) | 0.072595 / 0.023109 (0.049485) | 0.307224 / 0.275898 (0.031326) | 0.337244 / 0.323480 (0.013764) | 0.005296 / 0.007986 (-0.002690) | 0.003325 / 0.004328 (-0.001003) | 0.064589 / 0.004250 (0.060339) | 0.056369 / 0.037052 (0.019316) | 0.310829 / 0.258489 (0.052340) | 0.345563 / 0.293841 (0.051722) | 0.030551 / 0.128546 (-0.097995) | 0.008519 / 0.075646 (-0.067127) | 0.286368 / 0.419271 (-0.132903) | 0.052498 / 0.043533 (0.008966) | 0.308735 / 0.255139 (0.053596) | 0.329234 / 0.283200 (0.046034) | 0.022588 / 0.141683 (-0.119095) | 1.453135 / 1.452155 (0.000981) | 1.525956 / 1.492716 (0.033239) |

### 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) | 0.199417 / 0.018006 (0.181410) | 0.454621 / 0.000490 (0.454131) | 0.004928 / 0.000200 (0.004728) | 0.000079 / 0.000054 (0.000025) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.028436 / 0.037411 (-0.008975) | 0.083722 / 0.014526 (0.069196) | 0.095162 / 0.176557 (-0.081395) | 0.153434 / 0.737135 (-0.583702) | 0.099480 / 0.296338 (-0.196859) |

### 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) | 0.384647 / 0.215209 (0.169438) | 3.838406 / 2.077655 (1.760751) | 1.891267 / 1.504120 (0.387148) | 1.751432 / 1.541195 (0.210238) | 1.737443 / 1.468490 (0.268953) | 0.487758 / 4.584777 (-4.097019) | 3.635925 / 3.745712 (-0.109787) | 5.208718 / 5.269862 (-0.061144) | 3.029374 / 4.565676 (-1.536302) | 0.057613 / 0.424275 (-0.366662) | 0.007177 / 0.007607 (-0.000430) | 0.455596 / 0.226044 (0.229552) | 4.559969 / 2.268929 (2.291040) | 2.325321 / 55.444624 (-53.119303) | 2.034924 / 6.876477 (-4.841552) | 2.163869 / 2.142072 (0.021796) | 0.583477 / 4.805227 (-4.221750) | 0.132870 / 6.500664 (-6.367795) | 0.059618 / 0.075469 (-0.015851) |

### 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) | 1.263751 / 1.841788 (-0.578037) | 19.740004 / 8.074308 (11.665696) | 14.410980 / 10.191392 (4.219588) | 0.170367 / 0.680424 (-0.510057) | 0.018225 / 0.534201 (-0.515976) | 0.390101 / 0.579283 (-0.189182) | 0.404298 / 0.434364 (-0.030066) | 0.455295 / 0.540337 (-0.085043) | 0.621179 / 1.386936 (-0.765757) |

</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) | 0.006580 / 0.011353 (-0.004773) | 0.004078 / 0.011008 (-0.006930) | 0.065842 / 0.038508 (0.027334) | 0.074494 / 0.023109 (0.051385) | 0.403644 / 0.275898 (0.127746) | 0.430204 / 0.323480 (0.106724) | 0.005343 / 0.007986 (-0.002643) | 0.003366 / 0.004328 (-0.000963) | 0.064858 / 0.004250 (0.060607) | 0.056252 / 0.037052 (0.019200) | 0.412556 / 0.258489 (0.154067) | 0.434099 / 0.293841 (0.140258) | 0.031518 / 0.128546 (-0.097028) | 0.008543 / 0.075646 (-0.067104) | 0.071658 / 0.419271 (-0.347613) | 0.049962 / 0.043533 (0.006430) | 0.398511 / 0.255139 (0.143372) | 0.415908 / 0.283200 (0.132708) | 0.025011 / 0.141683 (-0.116672) | 1.492350 / 1.452155 (0.040195) | 1.552996 / 1.492716 (0.060280) |

### 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) | 0.204971 / 0.018006 (0.186964) | 0.439965 / 0.000490 (0.439475) | 0.002071 / 0.000200 (0.001872) | 0.000084 / 0.000054 (0.000029) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.031673 / 0.037411 (-0.005738) | 0.087529 / 0.014526 (0.073004) | 0.099882 / 0.176557 (-0.076675) | 0.156994 / 0.737135 (-0.580141) | 0.101421 / 0.296338 (-0.194918) |

### 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) | 0.407480 / 0.215209 (0.192271) | 4.069123 / 2.077655 (1.991468) | 2.081288 / 1.504120 (0.577169) | 1.920367 / 1.541195 (0.379172) | 1.981053 / 1.468490 (0.512563) | 0.481995 / 4.584777 (-4.102782) | 3.546486 / 3.745712 (-0.199226) | 5.133150 / 5.269862 (-0.136712) | 3.056444 / 4.565676 (-1.509232) | 0.056650 / 0.424275 (-0.367625) | 0.007746 / 0.007607 (0.000139) | 0.490891 / 0.226044 (0.264847) | 4.902160 / 2.268929 (2.633232) | 2.564726 / 55.444624 (-52.879899) | 2.234988 / 6.876477 (-4.641489) | 2.387656 / 2.142072 (0.245583) | 0.576315 / 4.805227 (-4.228912) | 0.132065 / 6.500664 (-6.368599) | 0.060728 / 0.075469 (-0.014741) |

### 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) | 1.370568 / 1.841788 (-0.471220) | 19.883159 / 8.074308 (11.808851) | 14.442066 / 10.191392 (4.250674) | 0.150119 / 0.680424 (-0.530305) | 0.018359 / 0.534201 (-0.515842) | 0.394128 / 0.579283 (-0.185155) | 0.411697 / 0.434364 (-0.022667) | 0.460580 / 0.540337 (-0.079757) | 0.608490 / 1.386936 (-0.778446) |

</details>
</details>

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

merging now if you don't mind - this way I can make a patch release