Add promptfoo to AI Observability and Monitoring tools

[mldangelo]

Summary

Added promptfoo to the AI Observability and Monitoring section in the favorite AI tools list.

promptfoo is an open-source framework for testing and evaluating LLM applications. It helps developers:

• Compare prompts and models
• Test RAG architectures
• Catch regressions and security vulnerabilities with CI/CD integration
• Red team LLM agents with multi-turn attack techniques

Used by over 250K developers and featured in OpenAI Build Hours and Anthropic's prompt evaluation courses.

Thanks for maintaining this guide!