docs: pdfdocs: Switch default CJK font to KR variants

xeCJK is enabled in Table of Contents (TOC) so that translations.pdf
built by top-level "make pdfdocs" can have its TOC typeset properly.

This causes quotation marks and apostrophe symbols appear too wide in
Latin-script docs.
This is because (1) Sphinx converts ASCII symbols into multi-byte
UTF-8 ones in LaTeX and (2) in the SC variant of "Noto CJK" font
families, those UTF-8 symbols have full-width glyph.

The KR variant of the font families has half-width glyph for those
symbols and TOC pages should look nicer when it is used instead.

Switch the default CJK font families to the KR variant and teach
xeCJK of those symbols' widths.
To compensate the switch, teach xeCJK of the width in the SC and
TC variants.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/0c8ea878-0a6f-ea01-ab45-4e66c5facee9@gmail.com


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