Add docs for enabling GitLab Container Registry for GitLab version <12.5

Link the doc and describe what is wrong with it.

https://docs.gitlab.com/ee/administration/packages/container_registry.html#enable-the-container-registry https://docs.gitlab.com/12.6/ee/administration/packages/container_registry.html

GitLab self-managed users running Omnibus version 12.5 and older, and all versions of Docker GitLab, can't find instructions documentation on how to enable GitLab registry.

In certain situations, (like this), admins read on and follow instructions for Installations from source instructions that suggest editing .yml files.