What does this MR do?
Document thoroughly how CI cache works.
What are the relevant issue numbers?
Closes #32515 (moved),#18945 (moved), gitlab-org/gitlab-ci-multi-runner#1621
TODOs
-
Consolidate info from https://gitlab.com/gitlab-org/gitlab-ce/issues/32515 -
Consolidate info from https://gitlab.com/gitlab-org/gitlab-runner/issues/336 -
Expose document to ci/README.md -
Document default cache:keyhttps://gitlab.com/gitlab-org/gitlab-ce/issues/18945