docs/qapidoc: add QMP highlighting to annotated qmp-example blocks

docs/qapidoc: add QMP highlighting to annotated qmp-example blocks

For any code literal blocks inside of a qmp-example directive, apply and
enforce the QMP lexer/highlighter to those blocks.

This way, you won't need to write:

```
.. qmp-example::
   :annotated:

   Blah blah

   .. code-block:: QMP

      -> { "lorem": "ipsum" }
```

But instead, simply:

```
.. qmp-example::
   :annotated:

   Blah blah::

     -> { "lorem": "ipsum" }
```

Once the directive block is exited, whatever the previous default
highlight language was will be restored; localizing the forced QMP
lexing to exclusively this directive.

Note, if the default language is *already* QMP, this directive will not
generate and restore redundant highlight configuration nodes. We may
well decide that the default language ought to be QMP for any QAPI
reference pages, but this way the directive behaves consistently no
matter where it is used.

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240717021312.606116-5-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>