Document deployments and rollbacks of BlueAPI with a scratch folder

Deploying a new version of BlueAPI is currently a bit fiddly, and would benefit from a how to guide, or potentially from a slight change into out setup-scratch works.

A few points:
- BlueAPI errors if the scratch path doesn't exist - maybe it should create the scratch folder if it doesn't already exist?
- The scratch folder needs to have extra permissions for setup-scratch to be successful (I needed to run `chmod o+wrX -R`). If BlueAPI is in charge of creating scratch then this can be done automatically, otherwise it should be documented
- It would be neat if rolling back a BlueAPI version would also roll back the versions of dodal and the plan repo. We should document whether or not this is a planned feature, as well as the current approach as to how to roll everything back

## Acceptance Criteria
- Above bullet points are addressed