docs/ja_JP: Convert SubmitChecklist into reST with belated updates

docs/ja_JP: Convert SubmitChecklist into reST with belated updates

This is a brand-new translation against
commit 2783096fb1dd ("docs: submit-checklist: Expand on build tests
against different word sizes"), rather than an update of
ja_JP/SubmitChecklist, which has never updated since 2008 except for
trivial changes not involving translation.

As we now have two reST contents under the ja_JP translation,
to avoid duplicated boiler plates, split out ja_JP's own disclaimer
part into a new section and put a reference to it at the beginning of
each doc.

As there is no prospect of ja_JP to have a lot of translated docs,
keep those .rst files in the toctree of ja_JP/index.rst.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Reviewed-by: Tsugikazu Shibata <shibata@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250217060132.64670-1-akiyks@gmail.com