docs/zh_CN: add core-api workqueue.rst translation

This patch translates Documentation/core-api/workqueue.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20210517132927.3461185-1-siyanteng@loongson.cn


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