MAINTAINERS: Put kernel-doc under the "docs build machinery" section

MAINTAINERS: Put kernel-doc under the "docs build machinery" section

We never had a MAINTAINERS entry for the old kernel-doc script; add
the files for the new Python kernel-doc under "Sphinx documentation
configuration and build machinery", as the most appropriate
subsection.

Mauro has kindly volunteered to help with maintenance/review
of this area of the codebase, so add him as a maintainer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-id: 20250814171324.1614516-9-peter.maydell@linaro.org