Document renovatebot and GitLab releases

Lift the contents of https://gitlab.developers.cam.ac.uk/uis/devops/renovate-config#keeping-terraform-configurations-up-to-date-with-latest-release-of-a-web-app into a separate how-to document.

In particular document the 15 minute release cache.

added Developer Experience Documentation labels

assigned to @rjw57

changed the description

added workflowIn Progress label

mentioned in commit 8ef96f43

mentioned in merge request !421 (merged)

mentioned in commit f32d2082

closed with merge request !421 (merged)

closed with commit 8ef96f43