docs: add comprehensive RichView API definition

Add detailed API specification document defining all public interfaces
for the RichView module.

Public API includes:
- RichView: Main SwiftUI component with configuration modifiers
- RenderConfiguration: Comprehensive configuration options with presets
- RichViewEvent: Event system for user interactions
- RenderMetadata: Performance diagnostics and metrics
- RenderError: Typed error handling
- RichViewCache: Global cache management interface

Key features:
- Minimal public surface (only essential APIs exposed)
- Type-safe event handling with enums
- Flexible configuration with presets (default/compact/accessibility)
- Performance monitoring through metadata
- Thread-safe cache management
- Migration guide from HtmlView

Documentation includes:
- Complete API reference with doc comments
- 10+ usage examples covering common scenarios
- Performance optimization tips
- Testing support helpers
- SwiftUI preview helpers

Internal APIs clearly separated to maintain encapsulation while
documenting module structure for implementation reference.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: graycreate