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

Documentation/translations/zh_CN/index.rst

@@ -28,6 +28,7 @@
    mips/index
    iio/index
    riscv/index
+   core-api/index
 
 目录和表格
 ----------