docs/zh_CN: add core api translation to zh_CN index

This patch add zh_CN/core-api to zh_CN/index.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/f23ea90fe88a6ac34d29c6642abe9aceba7ccafb.1618568135.git.siyanteng@loongson.cn


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