New model sharing tutorial
This PR removes the old serialization guide which was a bit outdated and adds the new instructions to the current model sharing tutorial (rewritten in rst to have proper links to the rest of the docs).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)
# [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).