docs/sphinx: Add new qapi-doc Sphinx extension

Some of our documentation is auto-generated from documentation
comments in the JSON schema.

For Sphinx, rather than creating a file to include, the most natural
way to handle this is to have a small custom Sphinx extension which
processes the JSON file and inserts documentation into the rST
file being processed.

This is the same approach that kerneldoc and hxtool use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-8-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comment and doc string formatting tweaked, unused method dropped,
a few line breaks tweaked to follow PEP 8 more closely, MAINTAINERS
section QAPI updated]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index be0a8e6..ac3e4e4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2387,6 +2387,7 @@
 F: tests/test-visitor-serialization.c
 F: scripts/qapi-gen.py
 F: scripts/qapi/*
+F: docs/sphinx/qapidoc.py
 F: docs/devel/qapi*
 T: git https://repo.or.cz/qemu/armbru.git qapi-next
 
@@ -3151,6 +3152,7 @@
 S: Maintained
 F: docs/conf.py
 F: docs/*/conf.py
+F: docs/sphinx/
 
 Miscellaneous
 -------------