docs: Move ja_JP/howto.rst to ja_JP/process/howto.rst

Let Japanese translation of howto.rst in the language drop-down list
by moving it to the expected place.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <b175c52f-34ee-4753-b172-e57fee6fcc30@gmail.com>

Documentation/translations/ja_JP/index.rst

@@ -11,7 +11,7 @@
 .. toctree::
    :maxdepth: 1
 
-   howto
+   process/howto
 
 .. raw:: latex