Document recommended setup of debusine:environments collection and associated workflows

(This is a followup to a discussion we had in #1015 (comment 642746))

We should enhance the Debusine documentation with instructions on how to configure a "debusine:environments" collection with recommended values for the various variants to generate and so on.

So far we have this:

• debusine's variant=none, built with mmdebstrap --variant=important (or debootstrap)
• debusine's variant=sbuild, built with mmdebstrap --variant=buildd
• debusine's variant=autopkgtest, built with something custom (mmdebstrap --variant=important + dpkg-dev or whatever ci.debian.net actually does)