WIP: doc: reorganize the TOC

Hello,

This is, for now, an empty commit to create this MR to track the progress of our work on re-organizing the TOC of the documentation.

Our progress are on this hackmd for now https://hackmd.logilab.fr/G0JhSynZR167BqPCGkHw2g?both

And we want to inspire ourselves from django's documentation index organization: https://docs.djangoproject.com/en/3.0/ (in rst https://github.com/django/django/blob/master/docs/index.txt)

We'll probably attempt to continue working on that after the next CW RQ next Tuesday

Cheers,