Added missed Chinese tutorial link (#8563) (#8564)

Author: ShunchiZhang

docs/zh_cn/index.rst

@@ -15,6 +15,7 @@ Welcome to MMDetection's documentation!
 
    1_exist_data_model.md
    2_new_data_model.md
+   3_exist_data_new_model.md
 
 .. toctree::
    :maxdepth: 2