Iterate the contribution model with lessons learned from Nav and so it matches our delivery cycle

[kellylee-gds]

What

Update the contribution model following what we learned whilst delivering Navigation components. We also need to match it to our delivery cycle.

Also update the team playbook page.

Why

There may be iterations we want to make now we've tried the model out.

Who needs to work on this

Kelly, Charlotte, Trang

Who needs to review this

A few folks from the team.

Done when