Introduce the 'custom containers' in the Foreword

The foreword should document the custom containers used throughout the book, so readers know what to expect when they encounter them.

A small table like :

| Container | Purpose (examples)|
|-----------|---------|
| `TIP` | Useful tips and best practices |
| `WARNING` | Important caveats or things to be careful about |
| `DANGER` | Critical warnings, e.g. about hardware damage or irreversible behavior |
| `DETAILS` | Collapsible supplementary information |

maybe previewing already the colors etc