docs/qapidoc: add preamble() method

docs/qapidoc: add preamble() method

This method adds the options/preamble to each definition block. Notably,
:since: and :ifcond: are added, as are any "special features" such as
:deprecated: and :unstable:.

If conditionals, if attached to special features, are currently
unhandled in this patch and will be addressed at a future date. We
currently do not have any if conditionals attached to special features.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-44-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>