Add synthetic demo data notebooks and documentation

[hiydavid]

Summary

• Add six standalone Databricks demo-data notebooks for banking, healthcare, retail, SaaS churn, talent advisory, and wind turbine maintenance
• Harden notebook setup with required catalog input, safe overwrite behavior, pinned Faker installs, and consistent end-of-run guidance
• Document the demo datasets and usage in notebooks/demo-data/README.md and add a pointer from the main README.md

Testing

• Static validation passed for all six notebooks with ast.parse
• Verified the updated notebook text for pinned Faker, required catalog handling, overwrite widget usage, and removal of the stale reference