docs/qapi-domain: add qapi:command directive

docs/qapi-domain: add qapi:command directive

This commit adds a stubbed version of QAPICommand that utilizes the
QAPIObject class, the qapi:command directive, the :qapi:cmd:
cross-reference role, and the "command" object type in the QAPI object
registry.

They don't do anything *particularly* interesting yet, but that will
come in forthcoming commits.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-14-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>