docs: fs: cifs: convert to ReST and add to admin-guide book

The filenames for cifs documentation is not using the same
convention as almost all Kernel documents is using. So,
rename them to a more appropriate name. Then, manually convert
the documentation files for CIFS to ReST.

By doing a manual conversion, we can preserve the original
author's style, while making it to look more like the other
Kernel documents.

Most of the conversion here is trivial. The most complex one was
the README file (which was renamed to usage.rst).

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>