docs/zh_CN: Add rbtree Chinese translation

Translate core-api/rbtree.rst into Chinese.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20211225015146.1535-1-tangyizhou@huawei.com


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