[Docs model summaries] Add pegasus to docs
Stiched together a short model summary for Pegasus. Would be great if @sshleifer and @sgugger can take a look :-) 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6640?src=pr&el=h1) Report
> Merging [#6640](https://codecov.io/gh/huggingface/transformers/pull/6640?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d0e42a7bed3de9271ae39c575d7eeb54cf985921?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/6640/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6640?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6640      +/-   ##
==========================================
- Coverage   79.14%   79.12%   -0.03%     
==========================================
  Files         156      156              
  Lines       28248    28248              
==========================================
- Hits        22358    22351       -7     
- Misses       5890     5897       +7     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6640?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6640/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `80.70% <0.00%> (-2.01%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6640/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (+0.25%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6640?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/6640?src=pr&el=footer). Last update [d0e42a7...773081d](https://codecov.io/gh/huggingface/transformers/pull/6640?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I guess we could in general link the model summaries to the model doc as well or vice-versa for each model? <|||||>We also should probably automatically link each model card to its doc

In the meantime, I think duplication is fine:)