docs/zh_CN: add core-api xarray translation

Translate Documentation/core-api/xarray.rst into Chinese

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/2a125bcb3220e7c1b72ae87bcad1b225dd950338.1634358018.git.siyanteng@loongson.cn


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