docs: conf.py: only use CJK if the font is available

If we try to build a book with asian characters with XeLaTeX
and the font is not available, it will produce an error.

So, instead, add a logic at conf.py to detect if the proper
font is installed.

This will avoid an error while building the document, although
the result may not be readable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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