docs/qapi-domain: add :deprecated: directive option

docs/qapi-domain: add :deprecated: directive option

Although "deprecated" is a feature (and *will* appear in the features
list), add a special :deprecated: option to generate an eye-catch that
makes this information very hard to miss.

The forthcoming Transmogrifier in qapidoc.py will add this option
whenever it detects that the features list attached to a definition
contains the "deprecated" entry.

P.S., I outsourced the CSS ;)

Signed-off-by: Harmonie Snow <harmonie@gmail.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-24-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>