Create scenario walkthrough documents

[kiyeonjeon21]

Priority: P1

README describes scenarios but no standalone step-by-step walkthroughs exist.

What to do

Create docs/scenarios/ with:

• batch-analytics.md — dbt + PostgreSQL + Metabase
• streaming-pipeline.md — Kafka + ClickHouse + Superset
• lakehouse.md — Iceberg + Trino + MinIO
• cdc-pipeline.md — Debezium + Kafka + ClickHouse

Each doc: prerequisites, step-by-step commands, verification queries, cleanup.

Acceptance criteria

• 4 scenario documents created
• Each is self-contained (new user can follow from zero)
• Includes SQL queries to verify data at each stage
• Linked from README's Scenarios section