MAINTAINERS: Become the docs maintainer

It seems it's my turn to be the documentation maintainer for a bit.  My
plan is to work to ensure that docs patches don't fall through the cracks;
I assume most changes will continue to flow through subsystem-specific
trees.

Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>