docs(weakmap-weakset): add comprehensive concept page with tests

- Add 986-line documentation covering WeakMap and WeakSet fundamentals
- Include real-world use cases: private data, caching, memoization, tracking
- Add comparison tables for Map vs WeakMap and Set vs WeakSet
- Cover ES2023+ Symbol keys feature
- Add 46 passing Vitest tests for all code examples
- Complete 5-phase concept-workflow (SEO score: 30/30)