This commit adds comprehensive practical examples and documentation to help users get started with PyIceberg:

New Example Notebooks:
- csv_migration_example.ipynb: CSV to Iceberg migration strategies
- time_travel_example.ipynb: Time travel queries and snapshot management

New Documentation:
- practical-examples.md: Guide for running and using practical examples
- migration-guide.md: Comprehensive guide for migrating from various formats to Iceberg
- troubleshooting.md: Common issues and solutions for PyIceberg users

Updated Documentation:
- SUMMARY.md: Added new documentation files to the table of contents

These additions provide real-world examples and guidance for common PyIceberg use cases, making it easier for users to adopt and use PyIceberg effectively.

- Fixed duplicate heading in migration-guide.md (Validation -> Post-Migration Validation)
- Removed specific notebook references from documentation to avoid link issues
- Fixed Jupyter notebook schema validation by adding missing outputs field
- Fixed import organization in notebooks by moving all imports to top cell
- Removed duplicate imports from cleanup cells
- Fixed end-of-file formatting issues

All linting checks now pass.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>