[FLINK-22350][python][docs] Add documentation for user-defined window…

… in Python DataStream API

This closes #15702.

docs/content.zh/docs/dev/python/datastream/operators/_index.md

@@ -0,0 +1,23 @@
+---
+title: Operators
+bookCollapseSection: true
+weight: 21
+---
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->

docs/content.zh/docs/dev/python/datastream/operators.md → docs/content.zh/docs/dev/python/datastream/operators/overview.md

@@ -1,9 +1,9 @@
 ---
-title: "Operators"
-weight: 21
+title: "Overview"
+weight: 1
 type: docs
 aliases:
-  - /zh/dev/python/datastream-api-users-guide/operators.html
+    - /zh/dev/python/datastream-api-users-guide/operators.html
 ---
 <!--
 Licensed to the Apache Software Foundation (ASF) under one