Add model summary
This PR adds a high-level summary of all the models in the documentation.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=h1) Report
> Merging [#4789](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b9109f2de1c4f52967976dc840074a9d62713498&el=desc) will **increase** coverage by `0.40%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4789/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4789      +/-   ##
==========================================
+ Coverage   76.06%   76.46%   +0.40%     
==========================================
  Files         128      128              
  Lines       21498    21498              
==========================================
+ Hits        16353    16439      +86     
+ Misses       5145     5059      -86     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4789/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4789/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.20% <0.00%> (-0.16%)` | :arrow_down: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4789/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `56.68% <0.00%> (+28.02%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=footer). Last update [b9109f2...0e3789d](https://codecov.io/gh/huggingface/transformers/pull/4789?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Looks great :-) <|||||>Eh, forgot to finish my sentence linking to the pretrained models doc page at the beginning. No sure if I can link to part of the tables however, does restructured text can support that?<|||||>I doubt it. One link is OK I think