Document the prioritisation process so we can repeat it next year

[christopherthomasdesign]

What

Document all the steps we went through to re-prioritise our backlog with the community.

Part of https://github.com/alphagov/govuk-design-system/issues/2220

Why

So that we can repeat it next year more efficiently.

Who needs to work on this

Designers, content / technical writer?

Who needs to review this

The team

Done when

• [ ] Documentation written
• [ ] Reviewed
• [ ] Decide where to publish and publish the final version (open on github or just an internal doc?)

[christopherthomasdesign]

Made a start here: https://docs.google.com/document/d/18rEFhMLYzQ8ppCCkqnb7weH39ZoeCpG0jbJLfidWbdQ/edit