Migration guide for v3

[GCHQ-Developer-847]

Summary

Write a migration guide, including all the breaking changes.

💰 Use value

To help users migrate to v3.

Proof of concept

This ticket also involves using the guidance to migrate the test-app into using V3

[GCHQ-Developer-847]

Include about adding a theme if customers do not have one, and setting to light so they don't break apps

[MI6-255]

This could be started on for everything so far, but not completed until all changes done

[MI6-255]

Make sure not to mark this as done until it's fully up to date