docs/qapidoc: add transmogrifier stub

docs/qapidoc: add transmogrifier stub

This commit adds a stubbed option to the qapi-doc directive that opts-in
to the new rST generator; the implementation of which will follow in
subsequent commits.

Once all QAPI documents have been converted, this option and the old
qapidoc implementation can be dropped.

Note that moving code outside of the try...except block has no impact
because the code moved outside of that block does not ever raise a
QAPIError.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-36-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>