zh_CN: Add translation for admin-guide/mm/index.rst

Translate Documentation/admin-guide/mm/index.rst into Chinese.
Update Documentation/admin-guide/index.rst.

Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/2d695dac05efc012b99fbc7525be65a421c7de03.1646899056.git.siyanteng@loongson.cn


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