Description & motivation

This is a stub, with more additions to come later. I'd like to add it in before office hours today.

Once we have a release candidate, I'm planning to add many much more to all v0.18.0 features. That looks like:

• opinionated discussion of the imagined use cases
• fill in any missing details in the comprehensive reference
• move this information to as many places as it should live (right now I've chosen to stick changes into as few files as possible)

Pre-release docs

Is this change related to an unreleased version of dbt?

• Yes
• No (if you're not sure, it's probably "No")

If yes, please change the base branch of this PR to next