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>