docs/qapidoc: add visit_feature() method

docs/qapidoc: add visit_feature() method

This adds a simple ":feat name: lorem ipsum ..." line to the generated
rST document, so at the moment it's only for "top level" features.

Features not attached directly to a QAPI definition are not currently
handled! This is a small regression over the prior documentation
generator that will be addressed in a future patch.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-49-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>