docs: Add link to GitHub release notes (#1394)

Author: manuzhang

mkdocs/docs/SUMMARY.md

@@ -29,6 +29,7 @@
 - Releases
     - [Verify a release](verify-release.md)
     - [How to release](how-to-release.md)
+    - [Release Notes](https://github.com/apache/iceberg-python/releases)
 - [Code Reference](reference/)
 
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