Document creation of MR in infra repo for new products.
At the moment https://guidebook.devops.uis.cam.ac.uk/en/latest/notes/gcp-deployments/#how-we-use-this treats some gpc inputs as appearing from the aether, whereas we normally create via terraform in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-infra.