Comprehensive Tachikoma SDK documentation update

- Enhanced Quick Start with complete setup instructions and examples
- Documented all supported AI providers (OpenAI, Anthropic, Grok, Ollama, Google, Mistral, Groq)
- Added detailed architecture overview with package structure
- Expanded core features documentation with type-safe model selection
- Included comprehensive usage examples for text generation, streaming, conversations, and tools
- Added advanced features: usage tracking, multi-step execution, structured output
- Complete installation guide with Swift Package Manager setup
- Environment configuration for all supported providers
- Build and test instructions for all modules

Author: steipete