docs: restructure documentation for improved clarity and navigation

Reorganize documentation into user-focused, developer-focused, and deployment-focused sections.

**New structure:**
- Root: README.md (streamlined), QUICK_START.md, API_REFERENCE.md
- docs/user-guide/: configuration, API usage, integrations, troubleshooting
- docs/developer/: contributing, development setup, testing, architecture
- docs/deployment/: Docker deployment, production checklist, monitoring
- docs/reference/: environment variables, MCP tools, data formats

**Changes:**
- Streamline README.md from 831 to 469 lines
- Create QUICK_START.md for 5-minute onboarding
- Create API_REFERENCE.md as single source of truth for API
- Remove 9 outdated specification docs (v0.2.0 API design)
- Remove DOCKER_API.md (content consolidated into new structure)
- Remove docs/plans/ directory with old design documents
- Update CLAUDE.md with documentation structure guide
- Remove orchestration-specific references

**Benefits:**
- Clear entry points for different audiences
- No content duplication
- Better discoverability through logical hierarchy
- All content reflects current v0.3.0 API

docs/developer/CONTRIBUTING.md

@@ -0,0 +1,48 @@
+# Contributing to AI-Trader
+
+Guidelines for contributing to the project.
+
+---
+
+## Development Setup
+
+See [development-setup.md](development-setup.md)
+
+---
+
+## Pull Request Process
+
+1. Fork the repository
+2. Create feature branch: `git checkout -b feature/my-feature`
+3. Make changes
+4. Run tests: `pytest tests/`
+5. Update documentation
+6. Commit: `git commit -m "Add feature: description"`
+7. Push: `git push origin feature/my-feature`
+8. Create Pull Request
+
+---
+
+## Code Style
+
+- Follow PEP 8 for Python
+- Use type hints
+- Add docstrings to public functions
+- Keep functions focused and small
+
+---
+
+## Testing Requirements
+
+- Unit tests for new functionality
+- Integration tests for API changes
+- Maintain test coverage >80%
+
+---
+
+## Documentation
+
+- Update README.md for new features
+- Add entries to CHANGELOG.md
+- Update API_REFERENCE.md for endpoint changes
+- Include examples in relevant guides