docs: collapse navbar to section landing pages

Previous structure dumped every top-level toctree entry from index.rst
into the navbar, producing eight items including external URLs ("Github
and Issue Tracker", "Rust's API Docs", ...) that wrapped to two lines
each. Introduce user-guide/index.rst and contributor-guide/index.rst as
section landing pages with nested toctrees, then point index.rst at just
those two plus autoapi/index. The navbar now reads "User Guide",
"Contributor Guide", "API Reference" — three single-line entries. Move
the external links into the index.rst body where they're discoverable
without crowding navigation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/contributor-guide/index.rst

@@ -0,0 +1,28 @@
+.. 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.
+
+=================
+Contributor Guide
+=================
+
+Guides for contributors to the DataFusion in Python project.
+
+.. toctree::
+   :maxdepth: 2
+
+   introduction
+   ffi

docs/source/index.rst

@@ -52,47 +52,18 @@ Example
     df.show()
 
 
-.. _toc.links:
-.. toctree::
-   :hidden:
-   :maxdepth: 1
-   :caption: LINKS
+Further reading:
 
-   Github and Issue Tracker <https://github.com/apache/datafusion-python>
-   Rust's API Docs <https://docs.rs/datafusion/latest/datafusion/>
-   Code of conduct <https://github.com/apache/datafusion/blob/main/CODE_OF_CONDUCT.md>
-   Examples <https://github.com/apache/datafusion-python/tree/main/examples>
+* `Examples <https://github.com/apache/datafusion-python/tree/main/examples>`_ — runnable scripts demonstrating common patterns.
+* `GitHub repository and issue tracker <https://github.com/apache/datafusion-python>`_.
+* `Rust API docs <https://docs.rs/datafusion/latest/datafusion/>`_ for the underlying engine.
+* `Apache DataFusion code of conduct <https://github.com/apache/datafusion/blob/main/CODE_OF_CONDUCT.md>`_.
 
-.. _toc.guide:
-.. toctree::
-   :hidden:
-   :maxdepth: 1
-   :caption: USER GUIDE
-
-   user-guide/introduction
-   user-guide/basics
-   user-guide/data-sources
-   user-guide/dataframe/index
-   user-guide/common-operations/index
-   user-guide/io/index
-   user-guide/configuration
-   user-guide/distributing-work
-   user-guide/sql
-   user-guide/upgrade-guides
-   user-guide/ai-coding-assistants
-
-
-.. _toc.contributor_guide:
-.. toctree::
-   :hidden:
-   :maxdepth: 1
-   :caption: CONTRIBUTOR GUIDE
 
-   contributor-guide/introduction
-   contributor-guide/ffi
-
-.. _toc.api:
 .. toctree::
    :hidden:
    :maxdepth: 1
-   :caption: API
+
+   user-guide/index
+   contributor-guide/index
+   API Reference <autoapi/index>

docs/source/user-guide/index.rst

@@ -0,0 +1,38 @@
+.. 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.
+
+==========
+User Guide
+==========
+
+The user guide walks through installing DataFusion in Python, building queries
+with the DataFrame API or SQL, reading and writing data, and tuning execution.
+
+.. toctree::
+   :maxdepth: 2
+
+   introduction
+   basics
+   data-sources
+   dataframe/index
+   common-operations/index
+   io/index
+   configuration
+   distributing-work
+   sql
+   upgrade-guides
+   ai-coding-assistants