docs: complete basic chat application tutorial with progressive examples
Completely rewrite the basic chat application tutorial with a structured, step-by-step approach:
**Tutorial Documentation**:
- Restructure tutorial into 6 clear progressive steps
- Add proper setup instructions and prerequisites
- Include code examples for each step with explanations
- Improve clarity and flow from basic to advanced concepts
**Complete Working Example**:
- Add `examples/tutorial-basic-chat-app/` with full implementation
- Include separate files for each tutorial step (step3-step6)
- Add comprehensive README with usage instructions
- Support multiple execution modes via command-line arguments
**Progressive Implementation Steps**:
- Step 3: Basic single-shot LLM interaction
- Step 4: Interactive chat loop without memory
- Step 5: Chat with manual conversation memory management
- Step 6: Advanced chat using chains with automatic memory
**Features Added**:
- Go module setup with proper dependencies
- Error handling and graceful exit functionality
- Multiple chat implementations demonstrating different approaches
- Clear documentation linking tutorial to working code
- Support for running individual steps or complete implementation
The tutorial now provides a complete learning path from basic LLM usage to sophisticated conversation management using LangChainGo's chains and memory systems.
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