docs/qapi-domain: add "Features:" field lists

docs/qapi-domain: add "Features:" field lists

Add support for Features field lists. There is no QAPI-specific
functionality here, but this could be changed if desired (if we wanted
the feature names to link somewhere, for instance.)

This feature list doesn't have any restrictions, so it can be used to
document object-wide features or per-member features as deemed
appropriate. It's essentially free-form text.

The syntax for this field is:

:feat name: description
    description cont'd

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-17-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>