Documentation and outreach for STF M4

The overall theme of milestone 4 is "The Debian security team can rely on debusine for their 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 host a private suite.
• Reach out to the security team (#394 (closed))
• Deliver talk at Toulouse mini-debconf

(TODO: Expand checklist with other tasks identified.)