docs: Activate exCJK only in CJK chapters

Activating xeCJK in English and Italian-translation documents
results in sub-optimal typesetting with wide-looking apostrophes
and quotation marks.

The xeCJK package provides macros for enabling and disabling its
effect in the middle of a document, namely \makexeCJKactive and
\makexeCJKinactive.

So the goal of this change is to activate xeCJK in the relevant
chapters in translations.

To do this:

    o Define custom macros in the preamble depending on the
      availability of the "Noto Sans CJK" font so that those
      macros can be used regardless of the use of xeCJK package.

    o Patch \sphinxtableofcontents so that xeCJK is inactivated
      after table of contents.

    o Embed those custom macros in each language's index.rst file
      as a ".. raw:: latex" construct.

Note: A CJK chapter needs \kerneldocCJKon in front of its chapter
heading, while a non-CJK chapter should have \kerneldocCJKoff
below its chapter heading.

This is to make sure the CJK font is available to CJK chapter's
heading and ending page's footer.

Tested against Sphinx versions 2.4.4 and 4.0.2.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Tested-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/lkml/2061da0a-6ab1-35f3-99c1-dbc415444f37@gmail.com
Link: https://lore.kernel.org/r/83208ddc-5de9-b283-3fd6-92c635348ca0@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -371,6 +371,19 @@ if cjk_cmd.find("Noto Sans CJK SC") >= 0:
 	% This is needed for translations
         \\usepackage{xeCJK}
         \\setCJKmainfont{Noto Sans CJK SC}
+	% Define custom macros to on/off CJK
+	\\newcommand{\\kerneldocCJKon}{\\makexeCJKactive}
+	\\newcommand{\\kerneldocCJKoff}{\\makexeCJKinactive}
+	% To customize \sphinxtableofcontents
+	\\usepackage{etoolbox}
+	% Inactivate CJK after tableofcontents
+	\\apptocmd{\\sphinxtableofcontents}{\\kerneldocCJKoff}{}{}
+     '''
+else:
+    latex_elements['preamble']  += '''
+	% Custom macros to on/off CJK (Dummy)
+	\\newcommand{\\kerneldocCJKon}{}
+	\\newcommand{\\kerneldocCJKoff}{}
      '''
 
 # Fix reference escape troubles with Sphinx 1.4.x

Documentation/translations/index.rst

@@ -18,6 +18,10 @@ Translations
 Disclaimer
 ----------
 
+.. raw:: latex
+
+	\kerneldocCJKoff
+
 Translation's purpose is to ease reading and understanding in languages other
 than English. Its aim is to help people who do not understand English or have
 doubts about its interpretation. Additionally, some people prefer to read

Documentation/translations/it_IT/index.rst

@@ -4,6 +4,10 @@
 Traduzione italiana
 ===================
 
+.. raw:: latex
+
+	\kerneldocCJKoff
+
 :manutentore: Federico Vaga <federico.vaga@vaga.pv.it>
 
 .. _it_disclaimer:

Documentation/translations/ja_JP/index.rst

 .. raw:: latex
 
-        \renewcommand\thesection*
-        \renewcommand\thesubsection*
+	\renewcommand\thesection*
+	\renewcommand\thesubsection*
+	\kerneldocCJKon
 
 Japanese translations
 =====================

Documentation/translations/ko_KR/index.rst

 .. raw:: latex
 
-        \renewcommand\thesection*
-        \renewcommand\thesubsection*
+	\renewcommand\thesection*
+	\renewcommand\thesubsection*
+	\kerneldocCJKon
 
 한국어 번역
 ===========

Documentation/translations/zh_CN/index.rst

@@ -4,6 +4,7 @@
 
 	\renewcommand\thesection*
 	\renewcommand\thesubsection*
+	\kerneldocCJKon
 
 .. _linux_doc_zh: