Move registry arch doc from Omnibus to main project

Integrates Omnibus documentation about the container registry into the main GitLab documentation.

Related MRs:

• gitlab!70502 (merged)
• omnibus-gitlab!5609 (merged)
• !2139 (merged)

Related issues:

• omnibus-gitlab#6362 (closed)

Part of "Move all the Omnibus documentation to the gitlab-org/gitlab project" &5173