docs: Add comprehensive multi-backend testing design

Document complete strategy for testing DataJoint against MySQL and PostgreSQL:
- Architecture: Hybrid testcontainers + docker-compose approach
- Three testing modes: auto, docker-compose, single-backend
- Implementation phases with code examples
- CI/CD configuration for parallel backend testing
- Usage examples and migration path

Provides complete blueprint for Phase 2-4 implementation.

Related: #1338

Author: dimitri-yatsenko