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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

MAINTAINERS

@@ -5987,6 +5987,12 @@ L:	linux-doc@vger.kernel.org
 S:	Maintained
 F:	Documentation/translations/it_IT
 
+DOCUMENTATION/JAPANESE
+R:	Akira Yokosawa <akiyks@gmail.com>
+L:	linux-doc@vger.kernel.org
+S:	Maintained
+F:	Documentation/translations/ja_JP
+
 DONGWOON DW9714 LENS VOICE COIL DRIVER
 M:	Sakari Ailus <sakari.ailus@linux.intel.com>
 L:	linux-media@vger.kernel.org