Document artifact registry pushes

Document the use of https://gitlab.developers.cam.ac.uk/uis/devops/continuous-delivery/ci-templates/-/blob/master/auto-devops/artifact-registry.yml.

Probably requires that #190 (closed) be addressed first.

Here is an example of this job in use in the Digital Admission's Malware Scanner project - https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/admissions-portal/ap-backend/-/jobs/1053272