docs: kerneldoc-preamble.sty: Suppress extra spaces in CJK literal blocks

In zh_CN part of translations.pdf, there are several ASCII-art
diagrams whose vertical lines look sometimes jagged.
This is due to the interference between default settings of xeCJK
and fancyvrb (employed in sphinxVerbatim env), where extra space
is inserted between a latin char and a non-latin char when they
are next to each other (i.e., no explicit white space).

This issue can be suppressed by invoking \CJKsetecglue{} at the
beginning of every sphinxVerbatim enviornment.

\AtBeginEnvironment, provided by the etoolbox package, is useful in
this case.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240905050941.31439-1-akiyks@gmail.com