Create image scaling documentation

Being close to finish image scaling, I think it would be good to start documenting the feature more for admings and developers, to understand the architecture, its limitations, and how it is configured and monitored.

Topics to address

• Workhorse configuration (we have an settings example in gitlab.rb.template -- is that enough?)
• Ops configuration flag (we mention it in the runbook -- should we expose to self-managed admins as well?)
• Contributor docs (this is completely missing)