infra: add docs for cleaning up testpypi artifacts (#1855)

## Which issue does this PR close?

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- Closes #1837

## What changes are included in this PR?
Used the [`pypi-cleanup`](https://github.com/arcivanov/pypi-cleanup)
utility to bulk cleanup old artifacts of pyiceberg-core from testpypi
for #1837. The alternative is to do this manually in the UI for each
artifact.

Adding this as reminder / instruction when the nightly pipeline fails
again

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## Are these changes tested?

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Co-authored-by: Fokko Driesprong <fokko@apache.org>

Author: kevinjqliu

.github/workflows/release_python_nightly.yml

@@ -128,9 +128,36 @@ jobs:
       - name: List downloaded artifacts
         run: ls -R bindings/python/dist
       - name: Publish to TestPyPI
+        id: publish-testpypi
+        continue-on-error: true
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
           repository-url: https://test.pypi.org/legacy/
           skip-existing: true
           packages-dir: bindings/python/dist
           verbose: true
+      - name: Display error message on publish failure
+        if: steps.publish-testpypi.outcome == 'failure'
+        run: |
+          echo "::error::Failed to publish to TestPyPI"
+          echo ""
+          echo "⚠️  TestPyPI Publish Failed"
+          echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+          echo ""
+          echo "This may be due to TestPyPI storage limits."
+          echo "See: https://docs.pypi.org/project-management/storage-limits"
+          echo ""
+          echo "To resolve this issue, use the pypi-cleanup utility to clean up old TestPyPI artifacts:"
+          echo "https://pypi.org/project/pypi-cleanup/"
+          echo ""
+          echo "  uvx pypi-cleanup --package pyiceberg-core --host https://test.pypi.org/ \\"
+          echo "    --verbose -d 10 --do-it --username <username>"
+          echo ""
+          echo "Requirements:"
+          echo "  • Must be a maintainer for pyiceberg-core on TestPyPI"
+          echo "    (https://test.pypi.org/project/pyiceberg-core)"
+          echo "  • Requires TestPyPI password and 2FA"
+          echo "  • ⚠️  ONLY do this for TestPyPI, NOT for production PyPI!"
+          echo ""
+          echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+          exit 1