Add comprehensive DocC documentation

[loloop]

Summary

This PR adds complete DocC documentation for the DiffableUI framework, providing comprehensive guides, tutorials, and API reference documentation.

Documentation Structure

📚 Core Documentation

• CollectionItem protocol - Detailed usage examples and conformance guide
• CollectionSection protocol - Layout guidance and compositional layout patterns
• DiffableViewController - Complete lifecycle and usage documentation
• CollectionViewBuilder - Result builder documentation with examples

🚀 Getting Started Guide

• Installation instructions for Swift Package Manager
• Step-by-step tutorial for creating your first collection view
• Common patterns and data handling examples
• Pull-to-refresh and error handling patterns

📖 Advanced Guides

• Creating Custom Items - Building reusable custom collection items
• Creating Custom Sections - Custom layouts with UIKit's compositional layout
• Handling User Interaction - Taps, swipes, gestures, selection, and context menus
• Building a News Feed - Complete example with pagination, loading states, and error handling
• Creating a Photo Grid - Responsive grid with multi-selection and zooming
• Implementing Pagination - Various pagination strategies and performance best practices

Additional Files

• CLAUDE.md - Guidance file for future Claude Code instances working on this project
• docs.sh - Convenience script to build and open documentation locally

Test plan

• Documentation builds successfully with xcodebuild docbuild
• All guides include working code examples
• Cross-references between topics work correctly
• API documentation includes parameter descriptions and return values
• Code examples follow Swift and iOS best practices

Key Features

✅ Comprehensive Coverage - Documents all public APIs with examples
✅ Progressive Learning - From basic setup to advanced patterns
✅ Real-world Examples - Complete tutorials with practical use cases
✅ Performance Tips - Best practices for memory management and optimization
✅ Modern Swift - Uses async/await, result builders, and Swift 5.9 features

The documentation can be built and viewed using:

# Build documentation
xcodebuild docbuild -scheme DiffableUI -derivedDataPath .build

# Or use the convenience script
./docs.sh

🤖 Generated with Claude Code

[loloop]

@claude please review