MAINTAINERS: mention documentation maintainer entry profile

Since commit 53b7f3aa

 ("Add a maintainer entry profile for
documentation"), the documentation "subsystem" has a maintainer entry
profile, and it deserves to be mentioned in MAINTAINERS with a suitable
P: entry.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200815102658.12236-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>