Add model summary

[sgugger]

This PR adds a high-level summary of all the models in the documentation.

[codecov]

Codecov Report

Merging #4789 into master will increase coverage by 0.40%.
The diff coverage is n/a.

@@            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	Coverage Δ
src/transformers/file_utils.py	73.85% <0.00%> (-0.42%)	⬇️
src/transformers/modeling_tf_utils.py	87.20% <0.00%> (-0.16%)	⬇️
src/transformers/data/processors/squad.py	56.68% <0.00%> (+28.02%)	⬆️

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[patrickvonplaten]

Looks great :-)

[sshleifer]

LGTM
I would add back and forth links with pretrained_models.rst

[sgugger]

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?

[sshleifer]

I doubt it. One link is OK I think