docs/qapi-domain: add qapi:enum directive

docs/qapi-domain: add qapi:enum directive

Add the .. qapi:enum:: directive, object, and :qapi:enum:`name`
cross-reference role.

Add the :value name: field list for documenting Enum values.

Of note, also introduce a new "type" role that is intended to be used by
other QAPI object directives to cross-reference arbitrary QAPI type
names, but will exclude commands, events, and modules from
consideration.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-20-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>