MAINTAINERS: treat Documentation/maintainer as process docs

A handful of people got caught out by the recent changes in git
which changed the format of Message-ID and broke our recommended
applyhook for adding lore links.

This was fixed in the docs by commit 2bb19e74 ("Documentation:
update git configuration for Link: tag") but it seems like few people
have noticed. Add maintainer directory to the process entry so that
workflows@ gets CCed.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20230713230713.1505561-1-kuba@kernel.org>