docs: add comprehensive technical architecture documentation

Created ARCHITECTURE.md as a complete technical reference for the
Crawl4AI Docker server, replacing the stress test pipeline document
with production-grade documentation.

Contents:
- System overview with architecture diagrams
- Core components deep-dive (server, API, utils)
- Smart browser pool implementation details
- Real-time monitoring system architecture
- WebSocket implementation and fallback strategy
- Memory management and container detection
- Production optimizations and code review fixes
- Deployment guides (local, Docker, production)
- Comprehensive troubleshooting section
- Debug tools and performance tuning
- Test suite documentation
- Architecture decision log (ADRs)

Target audience: Developers maintaining or extending the system
Goal: Enable rapid onboarding and confident modifications

Author: unclecode