docs: move checktransupdate.py to tools/docs

docs: move checktransupdate.py to tools/docs

The checktranslate.py tool currently languishes in scripts/; move it to
tools/docs and update references accordingly.

Cc: Alex Shi <alexs@kernel.org>
Cc: Yanteng Si <si.yanteng@linux.dev>
Cc: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>