In order to have the MAINTAINERS file visible in the rendered ReST
output, this makes some small changes to the existing MAINTAINERS file
to allow for better machine processing, and adds a new Sphinx directive
"maintainers-include" to perform the rendering.

Features include:
- Per-subsystem reference links: subsystem maintainer entries can be
  trivially linked to both internally and external. For example:
  https://www.kernel.org/doc/html/latest/process/maintainers.html#secure-computing

- Internally referenced .rst files are linked so they can be followed
  when browsing the resulting rendering. This allows, for example, the
  future addition of maintainer profiles to be automatically linked.

- Field name expansion: instead of the short fields (e.g. "M", "F",
  "K"), use the indicated inline "full names" for the fields (which are
  marked with "*"s in MAINTAINERS) so that a rendered subsystem entry
  is more human readable. Email lists are additionally comma-separated.
  For example:

    SECURE COMPUTING
	Mail:	  Kees Cook <keescook@chromium.org>
	Reviewer: Andy Lutomirski <luto@amacapital.net>,
		  Will Drewry <wad@chromium.org>
	SCM:	  git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git

 seccomp
	Status:	  Supported
	Files:	  kernel/seccomp.c include/uapi/linux/seccomp.h
		  include/linux/seccomp.h tools/testing/selftests/seccomp/*
		  tools/testing/selftests/kselftest_harness.h
		  userspace-api/seccomp_filter
	Content regex:	\bsecure_computing \bTIF_SECCOMP\b

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>