MAINTAINERS: Add jsnow as maintainer for Sphinx documentation

Since I've just about rewritten the entirety of the QAPI documentation
system, it's probably fair that I be the contact point for if it goes
awry.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-64-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

MAINTAINERS

@@ -4325,6 +4325,7 @@ S: Orphan
 F: po/*.po
 
 Sphinx documentation configuration and build machinery
+M: John Snow <jsnow@redhat.com>
 M: Peter Maydell <peter.maydell@linaro.org>
 S: Maintained
 F: docs/conf.py