docs/zh_CN: Add zh_CN/admin-guide/lockup-watchdogs.rst

Add translation zh_CN/admin-guide/lockup-watchdogs.rst and link it to
zh_CN/admin-guide/index.rst while clean its todo entry.

Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Hailong Liu <liu.hailong6@zte.com.cn>
Link: https://lore.kernel.org/r/20210603145227.30956-1-liuhailongg6@163.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>