doc-rst: Programmatically render MAINTAINERS into ReST

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>