docs/sphinx: remove legacy QAPI manual generator

docs/sphinx: remove legacy QAPI manual generator

Thanks for your service!

Remove the old qapidoc and the option to enable the transmogrifier,
leaving the "transmogrifier" as the ONLY qapi doc generator. This in
effect also converts the QAPI test to use the new documentation
generator, too.

Update doc-good.txt output to match the new doc generator, which I
should've done exactly when we switched over to the transmogrifier, but,
uhh, oops!

Notes on the new format:
 1. per-member IFCOND documentation is missing. Known issue.
 2. Freeform documentation without a header is now copied through into
    the output. This is a bug fix.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250618165353.1980365-4-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Fixes: b61a4eb3f32 (docs/qapidoc: support header-less freeform sections)
[Tweak commit message to say it's a bug fix, add Fixes:]
Signed-off-by: Markus Armbruster <armbru@redhat.com>