docs/qapi-domain: add type cross-refs to field lists

docs/qapi-domain: add type cross-refs to field lists

This commit, finally, adds cross-referencing support to various field
lists; modeled tightly after Sphinx's own Python domain code.

Cross-referencing support is added to type names provided to :arg:,
:memb:, :returns: and :choice:.

:feat:, :error: and :value:, which do not take type names, do not
support this syntax.

The general syntax is simple:

:arg TypeName ArgName: Lorem Ipsum ...

The domain will transform TypeName into :qapi:type:`TypeName` in this
basic case, and also apply the ``literal`` decoration to indicate that
this is a type cross-reference.

For optional arguments, the special "?" suffix is used. Because "*" has
special meaning in rST that would cause parsing errors, we elect to use
"?" instead. The special syntax processing strips this character from
the end of any type name argument and will append ", optional" to the
rendered output, applying the cross-reference only to the actual type
name.

The intent here is that the actual syntax in doc-blocks need not change;
but e.g. qapidoc.py will need to process and transform "@arg foo lorem
ipsum" into ":arg type? foo: lorem ipsum" based on the schema
information. Therefore, nobody should ever actually witness this
intermediate syntax unless they are writing manual documentation or the
doc transmogrifier breaks.

For array arguments, type names can similarly be surrounded by "[]",
which are stripped off and then re-appended outside of the
cross-reference.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-28-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>