docs/qapi-domain: add qapi:module directive

docs/qapi-domain: add qapi:module directive

This adds the qapi:module directive, which just notes the current module
being documented and performs a nested parse of the content block, if
present.

This code is based pretty heavily on Sphinx's PyModule directive, but
with unnecessary features excised.

For example:

.. qapi:module:: block-core

   Hello, and welcome to block-core!
   =================================

   lorem ipsum, dolor sit amet ...

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-12-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>