docs/zh_CN: add core-api mm-api translation

Translate Documentation/core-api/mm-api.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/2c3d01adcdaf9ad393a9cfbb18c34d18afae6ed5.1631846923.git.siyanteng@loongson.cn


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