docs/zh_CN: Add staging/xz Chinese translation

Translate the following files into Chinese:

- Documentation/staging/xz.rst

Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Rui Li <me@lirui.org>
Link: https://lore.kernel.org/r/281c6e063212aa337cccf549b816eec62e87c090.1666328379.git.me@lirui.org


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