docs: reconstruct READMEs

jupyterhub/README.md

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+# JupyterHub
+
+Multi-user platform for interactive computing:
+
+- Collaborative Jupyter notebook environment
+- Integrated with Keycloak for OIDC authentication
+- Persistent storage for user workspaces
+- Support for multiple kernels and environments
+- Vault integration for secure secrets management
+
+See [JupyterHub Documentation](../docs/jupyterhub.md) for detailed setup and configuration.
+
+## Installation
+
+```bash
+just jupyterhub::install
+```
+
+## Access
+
+Access JupyterHub at `https://jupyter.yourdomain.com` and authenticate via Keycloak.
+
+## buunstack Package & SecretStore
+
+JupyterHub includes the **buunstack** Python package, which provides seamless integration with HashiCorp Vault for secure secrets management in your notebooks.
+
+### Key Features
+
+- 🔒 **Secure Secrets Management**: Store and retrieve secrets securely using HashiCorp Vault
+- 🚀 **Pre-acquired Authentication**: Uses Vault tokens created automatically at notebook spawn
+- 📱 **Simple API**: Easy-to-use interface similar to Google Colab's `userdata.get()`
+- 🔄 **Automatic Token Renewal**: Built-in token refresh for long-running sessions
+
+### Quick Example
+
+```python
+from buunstack import SecretStore
+
+# Initialize with pre-acquired Vault token (automatic)
+secrets = SecretStore()
+
+# Store secrets
+secrets.put('api-keys',
+    openai_key='sk-your-key-here',
+    github_token='ghp_your-token',
+    database_url='postgresql://user:pass@host:5432/db'
+)
+
+# Retrieve secrets
+api_keys = secrets.get('api-keys')
+openai_key = api_keys['openai_key']
+
+# Or get a specific field directly
+openai_key = secrets.get('api-keys', field='openai_key')
+```
+
+### Learn More
+
+For detailed documentation, usage examples, and API reference, see:
+
+[📖 buunstack Package Documentation](../python-package/README.md)
+
+## Custom Container Images
+
+JupyterHub uses custom container images with pre-installed data science tools and integrations:
+
+### datastack-notebook (CPU)
+
+Standard notebook image based on `jupyter/pytorch-notebook`:
+
+- **PyTorch**: Deep learning framework
+- **PySpark**: Apache Spark integration for big data processing
+- **ClickHouse Client**: Direct database access
+- **Python 3.12**: Latest Python runtime
+
+[📖 See Image Documentation](./images/datastack-notebook/README.md)
+
+### datastack-cuda-notebook (GPU)
+
+GPU-enabled notebook image based on `jupyter/pytorch-notebook:cuda12`:
+
+- **CUDA 12**: GPU acceleration support
+- **PyTorch with GPU**: Hardware-accelerated deep learning
+- **PySpark**: Apache Spark integration
+- **ClickHouse Client**: Direct database access
+- **Python 3.12**: Latest Python runtime
+
+[📖 See Image Documentation](./images/datastack-cuda-notebook/README.md)
+
+Both images are based on the official [Jupyter Docker Stacks](https://github.com/jupyter/docker-stacks) and include all standard data science libraries (NumPy, pandas, scikit-learn, matplotlib, etc.).