docs/zh_CN: add a translation for index

The original file has added a former intro in commit b51208d4
("docs: Tweak the top-level Sphinx page") and hence update the Chinese
version for it.

Signed-off-by: Hu Haowen <src.res@email.cn>
Link: https://lore.kernel.org/r/20210608075207.77812-1-src.res@email.cn


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