Document Self-Service Gateway

From https://gitlab.developers.cam.ac.uk/uis/devops/admin/-/issues/205 using the template from !53 (merged)

changed milestone to %DevOps Sprint 119

added to epic uis/devops&24 (closed)

added Documentation priority2 Medium workflowSprint Ready labels

changed time estimate to 1d

added workflowIn Progress label and removed workflowSprint Ready label

mentioned in merge request !65 (merged)

added workflowReview Required label and removed workflowIn Progress label

closed with merge request !65 (merged)

mentioned in commit 74c57b47

marked #78 (closed) as a duplicate of this issue

marked this issue as related to #78 (closed)