docs: collapse archived user guide versions behind a single Older Versions page

docs/source/user-guide/index.md

@@ -22,18 +22,16 @@ under the License.
 The Comet user guide covers installation, configuration, supported data sources, supported operators
 and expressions, and tuning advice for running Apache Spark with Comet acceleration.
 
-User guides are published for each release. The development snapshot tracks the upcoming release and
-may include features and fixes that are not yet generally available. Select a version from the
-navigation menu to view its guide.
+The **current stable release** is 0.16.x. Most users should start there. The **development snapshot**
+tracks the upcoming release and may include features and fixes that are not yet generally available.
+User guides for **older releases** are kept for reference under [Older Versions](older-versions).
 
 ```{toctree}
 :maxdepth: 2
-:caption: User Guides
+:caption: User Guide
 :hidden:
 
-0.17.0-SNAPSHOT <latest/index>
-0.16.x <0.16/index>
-0.15.x <0.15/index>
-0.14.x <0.14/index>
-0.13.x <0.13/index>
+0.16.x (current) <0.16/index>
+Development Snapshot <latest/index>
+Older Versions <older-versions>
 ```

docs/source/user-guide/older-versions.md

@@ -0,0 +1,31 @@
+<!--
+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.
+-->
+
+# Older Versions
+
+These user guides are kept for reference. They are not maintained: use the
+[current stable release](0.16/index) for production guidance.
+
+```{toctree}
+:maxdepth: 1
+
+0.15.x <0.15/index>
+0.14.x <0.14/index>
+0.13.x <0.13/index>
+```