docs: clarify pip index configuration during PyPI migration

[harshitethic]

Summary

• document that pip index-url configuration can override the default PyPI index
• show how to locate active pip configuration files with pip config list -v
• document updating a legacy pypi.python.org index URL to pypi.org/simple

Fixes #463

Validation

• Focused documentation-only change in source/guides/migrating-to-pypi-org.rst
• Verified the commit diff contains only the intended documentation addition

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📚 Documentation preview 📚: https://python-packaging-user-guide--2119.org.readthedocs.build/en/2119/