[Docs model summaries] Add pegasus to docs

[patrickvonplaten]

Stiched together a short model summary for Pegasus. Would be great if @sshleifer and @sgugger can take a look :-)

[codecov]

Codecov Report

Merging #6640 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            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	Coverage Δ
src/transformers/generation_tf_utils.py	80.70% <0.00%> (-2.01%)	⬇️
src/transformers/file_utils.py	82.41% <0.00%> (+0.25%)	⬆️

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[sgugger]

Looks great to me! Thanks for updating the model summary!

[sshleifer]

This is great. But the info is now in ~3 places.
Model card, pegasus.rst, and here.
That seems like a lot of places. Can we kill 1?

[patrickvonplaten]

I guess we could in general link the model summaries to the model doc as well or vice-versa for each model?

[julien-c]

We also should probably automatically link each model card to its doc

In the meantime, I think duplication is fine:)