Add model and doc badges
Add badges at each model for:
- the page with all community models
- the documentation of the model

Remove the manual doc links as a result. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4811?src=pr&el=h1) Report
> Merging [#4811](https://codecov.io/gh/huggingface/transformers/pull/4811?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/875288b344d2181b789746e27e7b5bc62df8cae1&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4811/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4811?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4811      +/-   ##
==========================================
- Coverage   76.18%   76.15%   -0.03%     
==========================================
  Files         128      128              
  Lines       21497    21497              
==========================================
- Hits        16377    16372       -5     
- Misses       5120     5125       +5     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4811?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4811/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.41% <0.00%> (-0.79%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4811/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.27% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4811?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/4811?src=pr&el=footer). Last update [875288b...8ccd73a](https://codecov.io/gh/huggingface/transformers/pull/4811?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great