docs: pdfdocs: Add conf.py local to translations for ascii-art alignment

Globally choosing "Noto Sans Mono CJK SC" would result in sub-optimal look
of literal blocks in Latin documents.
Therefore, localize the font choice to translations by adding conf.py under
Documentation/translations/.

The local conf.py is enabled when the command:

    make SPHINXDIRS=translations pdfdocs

is used to build the PDF.
Resulting translations.pdf (under Documentation/output/translations/pdf)
will have properly aligned ascii-art figures.

NOTE: There remain mis-aligned ascii-art figures in Korean translations.
This is due to the font designer's decision to assign slightly narrower
widths (920) to Hangul characters in "Noto Sans Mono CJK KR" than those
of Hanja (Hanzi/Kanji) characters (1000) [1].

[1]: https://github.com/googlefonts/noto-cjk/issues/17

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/79a876fc-a552-f972-4a1b-c6df4cbb36dd@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
