docs: doc-guide: remove the extension from .rst files

On almost all places, we're including ReST files without the
extension.

Let's remove the extension here as well, in order to use just
one standard.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>