docs: consolidate and clean up documentation

- Remove redundant files: AGENTS.md, INSTRUCTIONS.yaml, instructions.mdc
- Create comprehensive CONTRIBUTING.md with all development guidelines
- Add simple documentation index at docs/README.md
- Update main README to reference consolidated docs
- Remove implementation notes and release summaries
- Keep only essential, current documentation

docs/README.md

@@ -0,0 +1,25 @@
+# UnitForge Documentation
+
+## 📚 Documentation
+
+| Document | Description |
+|----------|-------------|
+| [**Main README**](../README.md) | Project overview, installation, and usage |
+| [**Contributing Guide**](../CONTRIBUTING.md) | Development setup and workflow |
+| [**Environment Configuration**](ENVIRONMENT.md) | Environment variable reference |
+| [**Docker Setup**](../docker/README.md) | Container development |
+| [**Scripts & Utilities**](../scripts/README.md) | Development tools |
+
+## 🚀 Quick Start
+
+**New users**: Start with the [Main README](../README.md)
+
+**Contributors**: Follow the [Contributing Guide](../CONTRIBUTING.md)
+
+**Configuration**: See [Environment Variables](ENVIRONMENT.md)
+
+## 🔗 External References
+
+- [systemd documentation](https://systemd.io/)
+- [FastAPI documentation](https://fastapi.tiangolo.com/)
+- [uv documentation](https://github.com/astral-sh/uv)