Add how-to for destroying a product via the factory repos etc.

https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-project-factory/-/issues/84 has raised the issue that there is no clear document describing which order all the things need to be destroyed in when tearing down/decommissioning a product. Add a how-to detailing these steps and test the process at the same time to ensure it works and avoids dependency issues between the cloud platform repos etc.