docs: pdfdocs: Add CJK-language-specific font settings

zh_TW, ko_KR, and ja_JP translations have their preferred glyph sets
different from that of zh_CN.

To switch CJK font in the middle of the translations, introduce custom
LaTeX macros listed below:

    \kerneldocBeginSC
    \kerneldocEndSC
    \kerneldocBeginTC
    \kerneldocEndTC
    \kerneldocBeginKR
    \kerneldocEndKR
    \kerneldocBeginJP
    \kerneldocEndJP

, and embed a pair of macros in each language's index.rst.

NOTE 1: Update of zh_TW/index.rst is deferred to a follow-up change.

NOTE 2: Custom macros added here do not imply \kerneldocCJK(on|off).
This is intentional.  For example, \kerneldocCJKoff needs to be
at the top of Italian translations' index.rst for the footer of
final zh_TW page to be properly typeset.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/31906baf-7490-68cd-7153-4e4d320fb9b0@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>