Add AI & LLM Testing section with promptfoo

[mldangelo]

Adds a new AI & LLM Testing section to the Software category.

As LLM-powered applications become more prevalent, testing frameworks specific to AI are increasingly important. This PR adds:

• promptfoo - Open-source framework for testing and red teaming LLM applications

promptfoo enables testers to:

• Compare prompts and evaluate model outputs systematically
• Test RAG architectures for accuracy and relevance
• Run multi-turn adversarial attacks to find jailbreaks and prompt injection vulnerabilities
• Integrate security testing into CI/CD pipelines

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