Update the AI feature development playbook

Problem to solve

It can be hard for engineers working on AI features to know the practical steps they need to take to at various stages of the development workflow. The AI feature development playbook has some information, but it's incomplete and does not necessarily provide the relevant information in an easy-to-follow, practical style.

Proposal

Rewrite the AI feature development playbook to provide:

1. A high-level summary of what's involved in AI feature development (design, development (including prompt eng), testing/eval, deployment/monitoring, etc)
2. Summaries of each phase, with links to detailed guides (some existing, some will need to be written)
3. Links to reference materials

We can link to existing documentation as much as possible, and create issues to fill in the gaps.