docs(extract-v2): add complete walkthrough cookbook notebook

Runnable Jupyter notebook covering all Extract V2 features:
- Basic extraction, complex multi-page document
- Citations with bounding box visualization
- Confidence scores with charts
- Agentic vs cost_effective comparison
- Per-page extraction, advanced options, job management

Uses SDK 2.0. Test files included (all public/synthetic).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: eli-stewart