docs/sphinx: add sphinx modules to include D-Bus documentation

docs/sphinx: add sphinx modules to include D-Bus documentation

Add a new dbus-doc directive to import D-Bus interfaces documentation
from the introspection XML. The comments annotations follow the
gtkdoc/kerneldoc style, and should be formatted with reST.

Note: I realize after the fact that I was implementing those modules
with sphinx 4, and that we have much lower requirements. Instead of
lowering the features and code (removing type annotations etc), let's
have a warning in the documentation when the D-Bus modules can't be
used, and point to the source XML file in that case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>