Reorganize documentation navbar
With the library containing so many models now, the documentation navigation bar was started to get unreadable, especially if someone was looking for a specific model. To make this cleaner I:
- removed the PACKAGE REFENCE caption (not happy about this but there is no way to have a caption with empty content :-( )
- added three captions to separate the documentation between main classes, models and internals
- sorted alphabetically the sections

Also, make the background color of the section headers a bit darker to make the distinction with the rest of the toc clearer.

The result can be checked [here](https://92916-155220641-gh.circle-artifacts.com/0/docs/_build/html/index.html).
Cool, thanks @sgugger