Ready: Document dvv_enabled

Response to #1056.

- [x] Bucket type
- [x] Buckets.md
- [x] Add DVV theory doc

**PLEASE NOTE**: This PR should not be merged until PRs #999 and #1051 are merged, as it was written in anticipation of the changes made in those PRs.
This sounds like a +1 for DVVs, but we don't seem to make a hard recommendation to use them in this document. Should we?