docs: netlabel: convert docs to ReST and rename to *.rst

Convert netlabel documentation to ReST.

This was trivial: just add proper title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>