Documentation and outreach for STF M5

The overall theme of milestone 5 is "Debian developers can schedule their own archive-wide tests/experiments without disrupting regular day-to-day work". We should make sure that the improvements made as part of this milestone are reflected in our documentation, and that the Debian community is aware of the new possibilities this offers.

• Document how to use Debusine to run common tests/experiments (e.g. rebuilds)
• Clean up reference documentation for collections (!1525 (merged))
• Clean up reference documentation for artifacts (!1569 (merged))
• Document how to implement a new workflow (#693 (closed))
• Document file stores (!1670 (merged))
• Reach out to the Debian developer community

(TODO: Expand checklist with other tasks identified.)