[docs] Add menu w/ links to other pages on hf.co
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2761?src=pr&el=h1) Report
> Merging [#2761](https://codecov.io/gh/huggingface/transformers/pull/2761?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/33d3072e1c54bcd235447b98c6dea1b4cb71234c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2761/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2761?src=pr&el=tree)

```diff
@@          Coverage Diff           @@
##           master   #2761   +/-   ##
======================================
  Coverage    75.1%   75.1%           
======================================
  Files          93      93           
  Lines       15249   15249           
======================================
  Hits        11452   11452           
  Misses       3797    3797
```



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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2761?src=pr&el=footer). Last update [33d3072...e6944e6](https://codecov.io/gh/huggingface/transformers/pull/2761?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>btw @LysandreJik mkdocs looks really cool :)<|||||>Yeah I really like mkdocs as well