docs/zh_CN: add core api protection keys translation

Translate Documentation/core-api/protection-keys.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/abdcd4f157062a529cfc50754689d2df26070592.1625797729.git.siyanteng@loongson.cn


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