docs/qapidoc: add :namespace: option to qapi-doc directive

docs/qapidoc: add :namespace: option to qapi-doc directive

Add a :namespace: option to the qapi-doc directive, which inserts a
qapi:namespace directive into the start of the generated document. This,
in turn, associates all auto-generated definitions by this directive
with the specified namespace.

The source info for these generated lines are credited to the start of
the qapi-doc directive, which isn't precisely correct, but I wasn't sure
how to get it more accurate without some re-parsing shenanigans.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250313044312.189276-7-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>