docs: Tweak the top-level Sphinx page

This will be the initial landing point for readers, so give them a bit of
introductory material.  Also split the TOC into area-specific chunks to
make the whole thing a bit more approachable.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Jonathan Corbet 2016-10-26 15:43:00 -06:00
commit b51208d41c