doc/design: platform v2, physical distributed architecture of the query/control layer

[aljoscha]

Rendered

Related to #23543

Motivation

Part of MaterializeInc/database-issues#6316

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:

[danhhz]

Seems like a good thing to work through! This option seems to me to have the most upside, but certainly has the most open questions. I think until those are resolved a bit more, hard to have an opinion on which is the best end state.

Also I think there's a world in which we evolve to here through something like the Multi-cluster-controllerd, Multi-clusterd Architecture, but it'd be good to flesh out how that would work early on.