docs: improve README for better onboarding and value proposition

- Reorganize features list into logical categories (Core, Performance, Advanced, etc.)
- Add 'Why PDOdb?' section with comparisons vs Raw PDO and Eloquent/Doctrine
- Simplify Quick Example (remove JSON, use SQLite for zero-setup)
- Add comprehensive '5-Minute Tutorial' with step-by-step guide
- Add FAQ section with 10 common questions and answers
- Add Migration Guide with examples from Raw PDO, Eloquent, Doctrine, Yii2
- Improve structure and readability for beginners
- Lower entry barrier while maintaining comprehensive documentation

Author: tommyknocker