docs/zh_CN: add balance translation

Translate .../vm/balance.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/f1e5a6bcbee3e34fe65a56ee185d9b44daf01cab.1641866889.git.siyanteng@loongson.cn


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