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>

Documentation/conf.py

@@ -341,6 +341,9 @@ latex_elements = {
         verbatimhintsturnover=false,
     ''',
 
+    # For CJK One-half spacing, need to be in front of hyperref
+    'extrapackages': r'\usepackage{setspace}',
+
     # Additional stuff for the LaTeX preamble.
     'preamble': '''
 	% Prevent column squeezing of tabulary.
@@ -404,9 +407,12 @@ latex_elements['preamble']  += '''
 	}
 	\\newCJKfontfamily[JPsans]\\jpsans{Noto Sans CJK JP}
 	\\newCJKfontfamily[JPmono]\\jpmono{Noto Sans Mono CJK JP}
+	% Dummy commands for Sphinx < 2.3 (no 'extrapackages' support)
+	\\providecommand{\\onehalfspacing}{}
+	\\providecommand{\\singlespacing}{}
 	% Define custom macros to on/off CJK
-	\\newcommand{\\kerneldocCJKon}{\\makexeCJKactive}
-	\\newcommand{\\kerneldocCJKoff}{\\makexeCJKinactive}
+	\\newcommand{\\kerneldocCJKon}{\\makexeCJKactive\\onehalfspacing}
+	\\newcommand{\\kerneldocCJKoff}{\\makexeCJKinactive\\singlespacing}
 	\\newcommand{\\kerneldocBeginSC}{%
 	    \\begingroup%
 	    \\scmain%
@@ -437,6 +443,8 @@ latex_elements['preamble']  += '''
 	    \\renewcommand{\\CJKttdefault}{JPmono}%
 	}
 	\\newcommand{\\kerneldocEndJP}{\\endgroup}
+	% Single spacing in literal blocks
+	\\fvset{baselinestretch=1}
 	% To customize \\sphinxtableofcontents
 	\\usepackage{etoolbox}
 	% Inactivate CJK after tableofcontents