[FLINK-5618] [docs] Add intial queryable state page

docs/dev/stream/queryable_state.md

@@ -0,0 +1,32 @@
+---
+title: "Queryable State"
+nav-parent_id: streaming
+nav-pos: 61
+---
+<!--
+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.
+-->
+
+* ToC
+{:toc}
+
+<div class="alert alert-warning">
+  <strong>Note:</strong> The client APIs for queryable state are currently in an evolving state and there are <strong>no guarantees</strong> made about stability of the provided interfaces. It is likely that there will be breaking API changes on the client side in the upcoming Flink versions.
+</div>
+
+Coming up...