Add Design and Implementation Documentation

The IMM defines three kinds of Design and Implementation Documentation:

- [ ] Up-to-date architecture & high level conceptual docs & diagrams
- [ ] Logical infrastructure security & ops views
- [ ] Constraints & extensibility (obtained from benchmarks & specs)

We should add as much as is appropriate for each of these. When picking these off, add individual work items for each, and reference them from here - this issue is primarily to track progress on getting up to speed on the IMM.