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

Leonardo Maldonado committed on Jan 6, 2026, 10:00 PM

Showing 3 changed files +1842 additions -0 deletions

A docs/beyond/concepts/intersection-observer.mdx +1041

A tests/beyond/observer-apis/intersection-observer/intersection-observer.dom.test.js +375

A tests/beyond/observer-apis/intersection-observer/intersection-observer.test.js +426