docs: pdfdocs: One-half spacing for CJK translations

CJK documents are much easier to read with a wider baseline stretch.
Applying onehalfspacing option of "setspace" package looks reasonable.

Note: \usepackage{setspace} needs to be before that of hyperref in the
preamble.
The 'extrapackages' key (available since Sphinx 2.3) is for this purpose.
For systems with Sphinx < 2.3, dummy commands of \onehalfspacing and
\singlespacing are provided instead.
One-half spacing is not effective in LaTeX sources generated by such
revisions of Sphinx.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/547a1f1c-0bb5-20a7-ea0e-951bfc3c3925@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>