On-disk regions design doc

[antiguru]

Design doc for on-disk-regions.

Rendered: https://github.com/antiguru/materialize/blob/on_disk_regions/doc/developer/design/00000000_on_disk_regions.md

Motivation

Part of MaterializeInc/database-issues#6848.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: