doc-rst: add boilerplate to customize c-domain

Add a sphinx-extension to customize the sphinx c-domain.  No functional
changes right yet, just the boilerplate code.

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
[ jc: coding-style tweak ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>