docs/zh_CN: add core-api kernel-api.rst translation

This patch translates Documentation/core-api/kernel-api.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/d79e2d1f37bae52ce6fce0efb2fa4a32a89165fb.1621239725.git.siyanteng@loongson.cn


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