docs/zh_CN: Add coding-style into docs build system

Tested by the command:

    make htmldocs

During the compiling process, zh_CN/coding-style.rst has no errors and
warnings generated, the generated html document has been checked.

Signed-off-by: Andy Deng <theandy.deng@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>