docs: add Links sidebar section for external references

Restore the "Links" sidebar heading that the previous site had —
GitHub and Issue Tracker, Rust API Docs, Code of Conduct, Examples.
Implemented as a second hidden toctree with :caption: Links so the
pydata-sphinx-theme sidebar renders the heading above the four
external URLs. Drop Code of Conduct from the Contributor Guide
toctree since it now lives under Links instead.

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

Author: timsaucer

docs/source/contributor-guide/index.rst

@@ -26,4 +26,3 @@ Guides for contributors to the DataFusion in Python project.
 
    introduction
    ffi
-   Code of Conduct <https://github.com/apache/datafusion/blob/main/CODE_OF_CONDUCT.md>

docs/source/index.rst

@@ -59,3 +59,13 @@ Example
    user-guide/index
    contributor-guide/index
    API Reference <autoapi/index>
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+   :caption: Links
+
+   GitHub and Issue Tracker <https://github.com/apache/datafusion-python>
+   Rust 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>