docs/qapi-domain: add namespaced index support

docs/qapi-domain: add namespaced index support

Generate an index-per-namespace for the QAPI domain. Due to a limitation
with Sphinx's architecture, these indices must be defined during setup
time and cannot be dynamically created on-demand when a namespace
directive is encountered.

Owing to that limitation, add a configuration value to conf.py that
specifies which QAPI namespaces we'll generate indices for.

Indices will be named after their namespace, e.g. the "QMP" namespace
will generate to "qapi-qmp-index.html" and can be referenced using
`qapi-qmp-index`.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250313044312.189276-9-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>