examples: Add Google Colab quickstart notebook

Create a Colab notebook under examples/ that demonstrates:

• pip install -e . or pip install from GitHub
• Training with configs/default.yaml on tiny data
• Evaluation with code-explainer eval
• Serving via cx-serve (optional)
• Link back to examples/README.md

Deliverables: examples/colab_quickstart.ipynb and a badge in README.md pointing to it.

Labels: good first issue, documentation