CI artifacts object storage documentation

Prepare admin documentation how to use Object Storage for Artifacts: https://gitlab.com/gitlab-org/gitlab-ce/issues/29203

We should document what is needed for: configuring, validating, migrating data.

MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1762

TODOs

• Refactor https://docs.gitlab.com/ee/administration/job_artifacts.html#storing-job-artifacts section. Make two subsections: ### Using local storage, ### Using object storage
• Mention migrations in blog post