Reorganize documentation
This PR does two things:
- reorganize the doc topics in five sections
- complete the model list in the index

Also I cut all lines at 119 chars (like the code) otherwise it's not readable in visual studio code (and I imagine other viewers), removed the stars from authors since it wasn't pointing to anything (can add them back but we should explain what they mean in that case) and made all authors list comma-separated with one last 'and'.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5064?src=pr&el=h1) Report
> Merging [#5064](https://codecov.io/gh/huggingface/transformers/pull/5064?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d5477baf7d87b9bdad386f2f317732b85277b06b&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5064/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5064?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5064      +/-   ##
==========================================
- Coverage   77.41%   77.36%   -0.06%     
==========================================
  Files         130      130              
  Lines       22023    22023              
==========================================
- Hits        17050    17037      -13     
- Misses       4973     4986      +13     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5064?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `76.35% <0.00%> (-2.30%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.57% <0.00%> (+0.31%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5064?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/5064?src=pr&el=footer). Last update [d5477ba...d7a3d5d](https://codecov.io/gh/huggingface/transformers/pull/5064?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
