docs/zh_CN: add core api kobject translation

This patch translates Documentation/core-api/kobject.rst into Chinese.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/20210517133748.3461357-1-siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

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

@@ -35,9 +35,13 @@ Todolist:
 
 在整个内核中使用的函数库。
 
-Todolist:
+.. toctree::
+   :maxdepth: 1
 
    kobject
+
+Todolist:
+
    kref
    assoc_array
    xarray