Move style_doc to extra_quality_checks

[sshleifer]

Previously, if you have a doc style error in a .rst file,

python utils/style_doc.py $(modified_py_files) --max_len 119;

wouldn't catch it

[sgugger]

Ideally there should be a modified_py_and_rst_files to speed up this check (and only apply it to modified files), but this works in the meantime. @stas00 if you want to do that last bit of optimization, let me know, otherwise I'll do that later.

[stas00]

Go for it, @sgugger. The modified files var is there, so it should be easy to apply it anywhere.

If you get stuck I'm here to help.