Update installation page and add contributing to the doc
This PR simplifies the installation page, adds the mention of TF/PT installation with one command only (for CPU) and adds a test that the installation was successful.

The tests mention is moved to CONTRIBUTING. The mention of the tokenization process for OpenAI GPT is moved to that model doc page.

It also adds the contributing guide to the documentation with a simlink (add to fix a few links to make it work).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5084?src=pr&el=h1) Report
> Merging [#5084](https://codecov.io/gh/huggingface/transformers/pull/5084?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7291ea0bff57a017e71b1ea8ec01ff19da298bf0&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5084/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5084?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5084      +/-   ##
==========================================
+ Coverage   77.24%   77.26%   +0.02%     
==========================================
  Files         133      133              
  Lines       22146    22146              
==========================================
+ Hits        17107    17112       +5     
+ Misses       5039     5034       -5     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5084?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5084/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.61% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5084/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.40% <0.00%> (+0.40%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5084/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.42% <0.00%> (+0.77%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5084?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/5084?src=pr&el=footer). Last update [7291ea0...e9e1ea6](https://codecov.io/gh/huggingface/transformers/pull/5084?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
