Add guidance to the playbook on how to run a benefit / risks analysis and how to share it openly with users

[kellylee-gds]

What

Write a guide to completing a benefit / risk analysis.

Why

To help with decision making when we encounter a tricky question.

Who needs to work on this

A product or delivery manager

Who needs to review this

Anyone

Done when

• [ ] draft completed
• [ ] draft reviewed
• [ ] published on playbook