0.2.0

docs: Migrate from GitHub Pages to GitBook

- Add .gitbook.yaml configuration
- Create comprehensive SUMMARY.md structure
- Rewrite README.md with modern formatting and examples
- Add new documentation pages:
  - quickstart.md - Getting started guide
  - functions.md - Complete functions documentation
  - changelog.md - Version history
  - roadmap.md - Future plans
  - contributing.md - Contribution guidelines
  - development.md - Architecture and development guide
  - installer.md - Installation documentation
- Update language.md with v0.2.0 features
- Remove old GitHub Pages config (_config.yml)
- Remove unused assets

GitBook provides better organization, search, and navigation.