feat: add comprehensive Clean Code concept page

- Add complete guide covering clean code principles for JavaScript
- Include sections on meaningful naming, small functions, DRY, side effects
- Add early returns, guard clauses, and comments best practices
- Add SOLID principles with code examples for all 5 principles
- Include newspaper analogy with ASCII diagram
- Add Key Takeaways, Test Your Knowledge Q&A section
- Add curated articles and videos (Ryan McDermott, Fireship, Uncle Bob)
- Reference Robert C. Martin's Clean Code book as foundational text
- Link to related concepts: pure-functions, modern-js-syntax, error-handling

Author: leonardomso