docs/sphinx: Add new qapi-doc Sphinx extension

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>