docs/qapi-domain: add qapi:namespace directive

docs/qapi-domain: add qapi:namespace directive

Add a new directive that marks the beginning of a QAPI "namespace", for
example; "QMP", "QGA" or "QSD". This directive will associate all
subsequent QAPI directives in a document with the specified
namespace. This does not change the visual display of any of the
definitions or index entries, but does change the "Fully Qualified Name"
inside the QAPI domain's object table. This allows for two different
"namespaces" to define entities with otherwise identical names -- which
will come in handy for documenting both QEMU QMP and the QEMU Storage
Daemon.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250313044312.189276-6-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>