docs: add Gemma 4 Colab notebook

[abetlen]

Summary

• Add a cleaned Gemma 4 12B Colab notebook under examples/colab/notebook.ipynb.
• Link the notebook from the Gemma 4 multimodal README table with an Open in Colab badge.

Validation

• Validated notebook JSON structure.
• Confirmed the notebook has no saved outputs or execution counts.