docs: add real-world use cases with full pipeline examples + gap analysis

8 production use cases: Aurora CDC→Iceberg, real-time ClickHouse/Druid
analytics, multi-tenant SaaS, GraphRAG knowledge graphs, data contracts,
IoT time-series at scale, data mesh with DataHub. Each with complete
YAML configs showing how the plugin solves the problem.

Gap analysis identifies 8 roadmap items based on 2026 production patterns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: intel352