docs/qapi-domain: add qapi:object directive

docs/qapi-domain: add qapi:object directive

Adds the .. qapi:object:: directive, object, and :qapi:obj:`name`
cross-referencing role. This directive is meant to document both structs
and unions.

As per usual, QAPI cross-referencing for types in the member field list
will be added in a forthcoming commit.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-23-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>