Add comprehensive architecture and layout documentation

[thomasnemer]

Summary

This PR adds three comprehensive technical documentation files to help developers understand and use the Prong UI framework effectively.

Documentation Added

1. docs/architecture.md (25KB, 756 lines)

Framework architecture guide covering:

• System architecture overview with ASCII diagrams
• Scene-based system design and lifecycle
• Component architecture and CRTP patterns
• Coordinate system with caching details (links to coordinate_system.md)
• Renderer abstraction and interface
• Hierarchical event system with flow diagrams
• Focus management system
• Platform abstractions (IWindow, IClipboard, IKeyboard)
• Build system integration and C++20 features

2. docs/layouts.md (31KB, 1,195 lines)

Comprehensive layout system guide including:

• FlexLayout - Flexbox-inspired with direction, justify, align, grow/shrink
• GridLayout - CSS Grid-inspired with rows, columns, gaps
• DockLayout - Docking panels (top, bottom, left, right, center)
• StackLayout - Simple vertical/horizontal stacking
• FlowLayout - Automatic wrapping layouts
• Visual ASCII diagrams for each layout type
• Nested layout examples and real-world use cases
• Performance tips and troubleshooting guide

3. docs/best_practices.md (27KB, 1,124 lines)

Practical guidelines covering:

• Scene organization and custom scene patterns
• Layout selection decision tree
• Component creation with ComponentBuilder
• Coordinate system best practices (local vs global)
• Event handling patterns and callbacks
• Resize behavior guidelines for responsive UIs
• Performance optimization tips
• Common pitfalls and testing practices

Key Features

✅ Comprehensive coverage of all major framework concepts
✅ Practical code examples throughout (verified against current API)
✅ Cross-references to existing docs (coordinate_system.md, component_builder.md)
✅ Visual aids (ASCII diagrams, flow charts, layout examples)
✅ Good/bad example comparisons
✅ Developer-focused (adapted from CLAUDE.md)

Checklist

• docs/architecture.md created covering all required sections
• docs/layouts.md created with comprehensive guide for all 5 layouts
• docs/best_practices.md created with practical guidance
• All documentation includes code examples
• Code examples are accurate and reflect current API
• Cross-references to other docs added
• Clear diagrams/ASCII art where helpful
• No references to non-existent features

Test Plan

Documentation can be reviewed for:

• Accuracy of API examples
• Clarity and completeness of explanations
• Proper cross-referencing
• Code example correctness

Closes #14