docs(event-delegation): add comprehensive concept page with tests

- Complete event delegation documentation covering:
  - What is event delegation and how it works
  - event.target vs event.currentTarget differences
  - Element.matches() and Element.closest() usage
  - Handling dynamic/future elements automatically
  - Performance benefits of delegation pattern
  - Common patterns: action buttons, tabs, accordions
  - Limitations (non-bubbling events, stopPropagation)
  - Common mistakes and how to avoid them

- Add 13 comprehensive tests covering all code examples
- Include 4 MDN references, 4 articles, 3 video resources
- SEO optimized with 30/30 score