docs(computed-property-names): add comprehensive concept page with tests

- Add complete concept page covering ES6 computed property names syntax
- Include technical deep-dive on ToPropertyKey(), evaluation order, and type coercion
- Cover Symbol keys comprehensively (iterator, toStringTag, toPrimitive, privacy patterns)
- Document computed methods, getters/setters, and real-world use cases
- Add edge cases section covering duplicate keys, __proto__ gotcha, and error handling
- Create 41 comprehensive tests covering all code examples
- Include 4 MDN references, 4 articles, and 4 video resources
- SEO optimized: 3,449 words, 90% SEO score