docs/translations: Skip CJK contents if suitable fonts not found

docs/translations: Skip CJK contents if suitable fonts not found

On systems without "Noto Sans CJK" fonts, CJK chapters in
translations.pdf are full of "TOFU" boxes, with a long build time and
a large log file containing lots of missing-font warnings.

Avoid such waste of time and resources by skipping CJK chapters when
CJK fonts are not available.

To skip whole chapters, change the definition of
\kerneldocBegin{SC|TC|KR|JP} commands so that they can have an argument
to be ignored.
This works as far as the argument (#1) is not used in the command.
In place of skipped contents, put a note on skipped contents at the
beginning of the PDF.

Change the call sites in index.rst of CJK translations accordingly.

When CJK fonts are available, existing command definitions with
no argument just work.  LaTeX engine will see additional pairs of
"{" and "}", which add a level of grouping without having any effect
on typesetting.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/3359ca41-b81d-b2c7-e437-7618efbe241d@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>