docs: 00-INDEX: add missing entries for documentation files/dirs

Several directories and individual files don't have entries at
00-INDEX. Add them, using, as reference, the initial text inside
the documentation file(s).

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>