MAINTAINERS: Add entry for DOCUMENTATION/JAPANESE

Due to the lack of an entry for the Japanese translation of
documentation, Kosuke Fujimoto needed to ask the status of
its maintenance [1].

Add an entry for DOCUMENTATION/JAPANESE as a sub-subsystem
under the DOCUMENTATION subsystem to make it easier for
another prospective contributor to know there is someone
who takes care of Japanese translation.

As a first step, install myself as a designated reviewer.
Hopefully, other interested members get accustomed to the
kernel-dev workflow and would get ready to be
(co-)maintainers/reviewers shortly.

[1]: https://lore.kernel.org/r/172fa015-26df-c978-853d-3aba67c581cc@gmail.com/

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Cc: Kosuke Fujimoto <fujimotokosuke0@gmail.com>
Link: https://lore.kernel.org/r/7689b502-8c34-85b1-54e0-7a3b5e3c2bd1@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>