Draft: add registry caching documentation

[marians]

What this PR does

• Add architecture page about registry in the management cluster
• Add tutorial on how to use that registry as a cache from a workload cluster

Context: https://github.com/giantswarm/roadmap/issues/3065

This will have to stay unmerged while team honeybadger has to figure out some details and improvements.

[uvegla]

I intend https://github.com/giantswarm/docs/pull/2249 as a replacement for this.