docs(intersection-observer): add comprehensive concept page with tests

- Complete 4,000+ word documentation covering Intersection Observer API
- Cover lazy loading, infinite scroll, scroll animations, sticky headers
- Include ASCII diagrams comparing scroll events vs IntersectionObserver
- Add 37 tests (23 concept tests + 14 DOM tests)
- Curate 10 quality resources (2 MDN, 4 articles, 4 videos)
- Include Key Takeaways, Test Your Knowledge, and Common Mistakes sections