Add cross references for the documents mentioned at HOWTO and
are under the Documentation/ directory, using the ReST notation.

It should be noticed that HOWTO also mentions the /README file.
We opted to not touch it, for now, as making it build on
Sphinx would require it to be moved to a Documentation/foo
directory.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>