MAINTAINERS: Add documentation files to the corresponding sections

MAINTAINERS: Add documentation files to the corresponding sections

A lot of files in the docs directory do not have a maintainer according to
our MAINTAINERS file, though they can be clearly associated with one of the
sections in there. Add the files now so that our scripts/get_maintainer.pl
script can output the right maintainer for them.

Message-Id: <20221212174841.201003-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>