Adjust order and content of zh_CN/index.rst to make it clear,
complete introductions and TODOLists.

Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/20210430122234.GA655@bobwxc.top


Signed-off-by: Jonathan Corbet <corbet@lwn.net>