docs-rst: doc-guide: split the kernel-documentation.rst contents

Having the kernel-documentation at the topmost level doesn't
allow generating a separate PDF file for it. Also, makes harder
to add extra contents. So, place it on a sub-dir.

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