docs: improve the capabilities/classification guide (anthropics#239)

Enhanced the classification cookbook with clearer explanations and better
pedagogical structure throughout:

- Added context sections before key code blocks (data loading, evaluation
  framework, random baseline, simple classifier, RAG, CoT)
- Included analysis of confusion matrices after each classification approach,
  explaining what the results reveal and motivating the next technique
- Added progressive accuracy tracking (10% → 70% → 94% → 97%) to show
  empirical improvement at each step
- Improved evaluation section with clearer motivation for Promptfoo and
  production-scale evaluation needs
- Added comprehensive Promptfoo results analysis explaining temperature
  effects and production recommendations
- Fixed evaluate() function to use max_workers and as_completed() for proper
  rate limit handling without artificial delays

These changes make the guide more action-oriented while building transferable
understanding of why each technique (RAG, chain-of-thought) improves
classification accuracy.

🤖 Generated with my best friend, [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: PedramNavid