New model sharing tutorial
Author: sgugger
Pull request: yes

This PR removes the old serialization guide which was a bit outdated and adds the new instructions to the current model sharing tutorial (rewritten in rst to have proper links to the rest of the docs).

Also it adds a link to the trainer tutorial recently merged in the quicktour.

Preview is [here](https://53867-155220641-gh.circle-artifacts.com/0/docs/_build/html/model_sharing.html)

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=h1) Report
> Merging [#5323](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5543b30aa6b52da3c8f7d9e525b0edc26226d717&el=desc) will **increase** coverage by `0.48%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5323/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5323      +/-   ##
==========================================
+ Coverage   77.42%   77.91%   +0.48%     
==========================================
  Files         138      138              
  Lines       24284    24284              
==========================================
+ Hits        18803    18921     +118     
+ Misses       5481     5363     -118     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5323/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5323/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.26% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5323/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.62% <0.00%> (+1.47%)` | :arrow_up: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5323/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `87.50% <0.00%> (+58.65%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5323/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.38% <0.00%> (+68.46%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=footer). Last update [5543b30...00ff8c3](https://codecov.io/gh/huggingface/transformers/pull/5323?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


very cool! Let's share it with the hugging face twitter account when it's live to get more people to share their models on the hub!

That would be now then, it's live in the [master docs](https://huggingface.co/transformers/master/model_sharing.html).