Documentation and outreach for STF M2

The overall theme of milestone 2 is "Debusine can automatically schedule QA tasks". 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 to simplify their workflow.

• Document the behavior of workers (!860 (merged))
  • e.g. process tasks one by one, what happens when worker is rebooted/process is stopped, manages cache of system images as per #316 (closed), etc.
• Blog post with progress (https://www.freexian.com/blog/debusine-0.3.0/)
• Update tutorial to cover incus backend setup (#361 (closed))
• Update getting-started tutorial to use workflows and collections (!859 (merged))