Add comprehensive project overview documentation

[Copilot]

This PR adds comprehensive documentation to help users and contributors better understand the Flutter AI Toolkit project structure, capabilities, and usage patterns.

What's Added

📋 PROJECT_OVERVIEW.md

A detailed technical overview covering:

• Project introduction and key features (multi-turn chat, streaming responses, voice input, etc.)
• Architecture breakdown with component relationships and provider system
• Comprehensive examples showcasing the 15+ demo applications
• Migration guide for the recent v0.9.0-0.10.0 Firebase AI consolidation
• Setup instructions and code examples for quick implementation
• Use cases and enterprise readiness details

📄 SUMMARY.md

A quick reference guide providing:

• Essential project statistics (9K+ lines of code, cross-platform support)
• Core features checklist with visual indicators
• Simplified architecture diagram
• One-command setup instructions
• Recent changes highlight

Why This Helps

The Flutter AI Toolkit is a sophisticated package with extensive capabilities, but newcomers may not immediately understand:

• The full scope of features available (beyond basic chat)
• How the pluggable architecture works with different AI providers
• The wealth of example applications demonstrating real-world usage
• Recent major changes in the Firebase AI migration

These documentation files provide both quick orientation for new users and comprehensive reference for developers implementing advanced features like AI agents with function calling, voice integration, and custom styling.

Content Highlights

• Production-ready: Emphasizes this is an official Flutter package with enterprise-grade quality
• Recent evolution: Documents the significant v0.9.0 migration to unified Firebase AI
• Rich examples: Highlights the 15+ example apps from basic chat to AI agents
• Architecture clarity: Explains the LlmProvider system and how to extend it
• Real-world focus: Shows practical use cases and implementation patterns

The documentation maintains consistency with the existing README.md while providing additional depth for users who want to fully understand and leverage the toolkit's capabilities.

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