Add comprehensive documentation for OpenObservation

[Kyle-Ye]

Summary

• Add comprehensive DocC documentation for the OpenObservation framework
• Update OpenObservationClient with extensive examples demonstrating all features
• Fix documentation warnings and clarify onChange one-shot behavior

Changes

Documentation Added

• Getting Started Guide: Complete tutorial for using OpenObservation with practical examples
• How Observation Works: Deep technical dive into the internals of the framework
• Performance Considerations: Detailed guide on optimization and best practices
• Main Documentation Page: Overview with proper symbol links and navigation

Example Updates

• Enhanced OpenObservationClient with comprehensive examples including:
  • Basic observable classes with @ObservationIgnored properties
  • Computed properties demonstration
  • Nested observables (ShoppingCart with CartItems)
  • Selective property observation
  • Multiple observation tracking scenarios
  • Clear comments explaining onChange one-shot behavior

Important Behavioral Clarifications

• Documented that withObservationTracking's onChange closure fires only once after the first property change
• Added examples showing how to re-establish observations for continuous tracking
• Updated troubleshooting section with common pitfalls

Test Plan

• Documentation builds without warnings using xcodebuild docbuild
• All examples in OpenObservationClient run successfully
• Tests pass with swift test
• Examples correctly demonstrate one-shot onChange behavior