docs/qapidoc: split old implementation into qapidoc_legacy.py

docs/qapidoc: split old implementation into qapidoc_legacy.py

This is being done primarily to be able to type check and delint the new
implementation without needing to worry about fixing up the old
implementation.

I'm adding the new implementation into the existing file instead of into
a new file so that when the dust settles, qapidoc.py will contain the
full history of development on this generative module.

This patch *should* be pure motion, give or take the import statements.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-37-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>