Update Hub and huggingface_hub docs

[osanseviero]

This PR does two things

• Updates the add a model guide based on feedback
• Does a general cleanup/update/simplification of huggingface_hub README.

[julien-c]

good improvements

(nit) Not a fan of the 80-char line wrapping as it makes subsequent diffs hard to read

[LysandreJik]

Was actually thinking of bumping up the black character limit to 119 like transformers, it greatly improves readability imo

[julien-c]

hmm those are .md files

[LysandreJik]

Looks good to me, thanks for completing the docs with the docstrings!

[LysandreJik]

hmm those are .md files

Yes, I was talking about the Python files, not the files here. Bouncing back on your comment that 80 characters isn't necessarily more readable!

[osanseviero]

I wrapped all the file since I realize a section of the file was wrapped and the other was not. The diff is easy to read in rich mode for markdown though, and for future PRs it should be easy if we stay that way.