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 3b4c963243b1 ("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:

  - f7ebe6b76940 ("docs: Activate exCJK only in CJK chapters")
  - 0afd4df0d16a ("docs: pdfdocs: Prevent column squeezing by tabulary")
  - 659653c9e546 ("docs: pdfdocs: Refactor config for CJK document")
  - e291ff6f5a03 ("docs: pdfdocs: Add CJK-language-specific font settings")
  - 7eb368cc319b ("docs: pdfdocs: Choose Serif font as CJK mainfont if possible")
  - 35382965bdd2 ("docs: pdfdocs: Preserve inter-phrase space in Korean translations")
  - 77abc2c230b1 ("docs: pdfdocs: One-half spacing for CJK translations")
  - 788d28a25799 ("docs: pdfdocs: Permit AutoFakeSlant for CJK fonts")
  - 29ac9822358f ("docs: pdfdocs: Teach xeCJK about character classes of quotation marks")
  - 7c5c18bdb656 ("docs: pdfdocs: Fix typo in CJK-language specific font settings")
  - aa872e0647dc ("docs: pdfdocs: Adjust \headheight for fancyhdr")
  - 8716ef413aa5 ("docs: pdfdocs: Tweak width params of TOC")
  - 66939df53948 ("docs: pdfdocs: Switch default CJK font to KR variants")
  - 7b686a2ea1e4 ("docs: pdfdocs: Enable CJKspace in TOC for Korean titles")
  - 5d9158e3c762 ("docs/translations: Skip CJK contents if suitable fonts not found")
  - b774cc46313b ("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>

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