gadget.rst: Enrich its ReST representation and add kernel-doc tag

The pandoc conversion is not perfect. Do handwork in order to:

- add a title to this chapter;
- use the proper warning and note markups;
- use kernel-doc to include Kernel header and c files;
- remove legacy notes with regards to DocBook;
- some other minor adjustments to make it better to read in
  text mode and in html.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>