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

Documentation/translations/zh_CN/core-api/index.rst

@@ -42,6 +42,7 @@
    kref
    assoc_array
    xarray
+   rbtree
 
 Todolist:
 
@@ -49,7 +50,6 @@ Todolist:
 
    idr
    circular-buffers
-   rbtree
    generic-radix-tree
    packing
    bus-virt-phys-mapping