docs/zh_CN: Link zh_CN/doc-guide to zh_CN/index.rst

Add zh_CN/doc-guide entry in zh_CN/index.rst
Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/8738b39c1b54e15477a937c861f114165a8c0648.1618295149.git.bobwxc@email.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/index.rst

@@ -18,6 +18,7 @@
 
    admin-guide/index
    process/index
+   doc-guide/index
    kernel-hacking/index
    filesystems/index
    arm64/index