MAINTAINERS: Add entry for Sphinx documentation infrastructure

Add a MAINTAINERS entry for Sphinx documentation infrastructure:
this doesn't cover actual content, only the machinery we use to
build the docs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-13-peter.maydell@linaro.org