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>