docs(getters-setters): add comprehensive concept page with tests

- Add full concept documentation (1,214 lines, ~4,200 words)
- Cover object literals, classes, Object.defineProperty() syntax
- Include vending machine analogy with ASCII diagram
- Document infinite recursion prevention pattern
- Add 5 common use cases with code examples
- Cover inheritance, performance, and JSON.stringify behavior
- Add 49 passing tests covering all concepts and edge cases
- SEO optimized: score 29/30 with featured snippet ready definition
- Include 11 external resources (4 MDN, 4 articles, 3 videos)