docs: pdfdocs: Choose Serif font as CJK mainfont if possible

"Noto Serif CJK SC" and its variants suit better with the roman font
of Latin letters.

On some distros such as Fedora, it is possible to partially install
"Noto Sans CJK" fonts.
So, if the Serif fonts are not found on the system, fall back to the
Sans fonts.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/5e454140-5d0c-35d4-8c31-3ffb2420793a@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>