docs/zh_CN: Refactoring the admin-guide directory index

The Todolist in the html document looks a mess, now give it a nice looking format.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/d410408ec13d6e9cff97da50a13d793a428e05cf.1646899089.git.siyanteng@loongson.cn


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