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

- Add complete Performance Observer API documentation covering:
  - Core Web Vitals measurement (LCP, CLS, INP, FCP, TTFB)
  - Resource, navigation, and paint timing
  - Long Tasks detection and custom marks/measures
  - Building Real User Monitoring (RUM) solutions
  - web-vitals library integration
- Add 24 tests covering all code examples
- Include Key Takeaways, Test Your Knowledge, and Related Concepts
- Add curated MDN references, articles, and videos