docs: pdfdocs: Pull LaTeX preamble part out of conf.py

docs: pdfdocs: Pull LaTeX preamble part out of conf.py

Quote from Jon's remark [1]:

    I do notice that Documentation/conf.py is getting large and
    unapproachable.  At some future point, it might be nice to pull
    all of the latex stuff out into a separate file where it won't
    scare people who stumble into it by accident.

Pull LaTeX preamble settings added since commit 6f2a5b35bee8 ("docs:
conf.py: adjust the LaTeX document output") out into
sphinx/kerneldoc-preamble.sty.

It will be copied to the build directory by the added
"latex_additional_files" setting in conf.py.

As a bonus, LaTeX/TeX code can be maintained without escaping backslashes.

To compensate the loss of change history in sphinx/kerneldoc-preamble.sty,
here is a list of changes made in conf.py:

  - 9966481fbd71 ("docs: Activate exCJK only in CJK chapters")
  - b38fac965b70 ("docs: pdfdocs: Prevent column squeezing by tabulary")
  - 123bf42c73d5 ("docs: pdfdocs: Refactor config for CJK document")
  - cbbd5dfdbcd2 ("docs: pdfdocs: Add CJK-language-specific font settings")
  - 1c3f8e4d0731 ("docs: pdfdocs: Choose Serif font as CJK mainfont if possible")
  - b94fcd8c79be ("docs: pdfdocs: Preserve inter-phrase space in Korean translations")
  - dac183fa8d3f ("docs: pdfdocs: One-half spacing for CJK translations")
  - c7d34c131fc2 ("docs: pdfdocs: Permit AutoFakeSlant for CJK fonts")
  - d20edb909ef4 ("docs: pdfdocs: Teach xeCJK about character classes of quotation marks")
  - f656087e2e33 ("docs: pdfdocs: Fix typo in CJK-language specific font settings")
  - e615474700eb ("docs: pdfdocs: Adjust \headheight for fancyhdr")
  - fa4f779794df ("docs: pdfdocs: Tweak width params of TOC")
  - 9e3f3cf02fdd ("docs: pdfdocs: Switch default CJK font to KR variants")
  - 32ec19662cf7 ("docs: pdfdocs: Enable CJKspace in TOC for Korean titles")
  - b60dc7ef1f09 ("docs/translations: Skip CJK contents if suitable fonts not found")
  - 7af670814048 ("docs: pdfdocs: Move CJK monospace font setting to main conf.py")

[1]: https://lore.kernel.org/all/87zgmr66cn.fsf@meer.lwn.net/

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/aaa9dca1-27c0-c414-77f3-c5587db0cc5b@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>