docs/zh_CN: Add userspace-api/index Chinese translation

Translate userspace-api/index.rst into Chinese, add it into
zh_CN/index.rst.

Signed-off-by: Rui Li <me@lirui.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/76b86e687034d5347e1e49c4acfc28e9c45abe47.1666185911.git.me@lirui.org


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