docs/zh_CN: Add zh_CN/admin-guide/sysrq.rst

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

Signed-off-by: Junhua Huang <huang.junhua@zte.com.cn>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20210910022531.243882-1-junhuahuangdream@163.com


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