Docs for container registry on <12.5 or no LE

What does this MR do?

Links instructions to enable Container Registry on GitLab instances that are not:

• using LetsEncrypt integration
• GitLab version 12.5 or newer

Closes !28713 (closed) Closes #212479 (closed)

Why?

GitLab users running <12.5 or not using letsencrypt are confused about how to enable Container registry, causing them to "read on", follow Installation from source instructions, and actually start editing yaml files on Omnibus installations (bad practice).

• https://forum.gitlab.com/t/gitlab-docker-registry-yaml-config/35600/2?u=gitlab-greg
• https://forum.gitlab.com/t/backup-skips-the-registry-part-and-says-disabled/35715/4?u=gitlab-greg

@sselhorn @axil