Add complete Apple-style DocC documentation for TUIKit framework:
## Documentation Structure
- Main landing page (TUIKit.md) with quick-start and topic navigation
- 8 guide articles: GettingStarted, ViewHierarchy, StateManagement, Theming, Appearance, Focus, Modifiers, Architecture
- 3 step-by-step tutorials: BuildYourFirstApp, BuildInteractiveMenu, BuildThemableUI
- Documentation catalog with proper Info.plist configuration
## Code Improvements
- Enhanced Box.swift documentation with clear container comparison
- Added comprehensive examples and usage patterns
- Explained appearance integration and sizing behavior
## Infrastructure
- GitHub Actions workflow (.github/workflows/docc.yml) for automatic DocC building
- Configured for deployment to GitHub Pages on push to main
- Uses xcrun docc build with proper artifact handling
## Documentation Includes
- API reference stubs for all major components
- Comprehensive guide articles with code examples
- Interactive tutorials following Apple's documentation style
- Cross-references using DocC syntax (``Type``)
- Best practices and design patterns explained
The documentation is ready for automated building and deployment to GitHub Pages.
