Add note on DevOps Google deployments
As noted in https://gitlab.developers.cam.ac.uk/uis/devops/raven/deploy-raven-core/-/merge_requests/1#note_102459, we have no place to refer people to about how our deployments work. Take the text from that document and the diagram and add it to the guidebook.
Blocks https://gitlab.developers.cam.ac.uk/uis/devops/raven/deploy-raven-core/issues/1