cluster spec sheet: Add source scaling scenario

[def-]

Depends on https://github.com/MaterializeInc/i2/pull/3047, so currently can't test

I only tested the commands in another Materialize instance and everything worked, but of course possible that I made some mistakes with porting them. @antiguru @ggevay Does this approach make sense? My goal is to get a table with the cluster size and how fast ingestions of various source types are. All of the tables are 50 million rows, so we can calculate rows/s and MB/s.

First green run: https://buildkite.com/materialize/release-qualification/builds/1039#019bd661-cce0-49a8-be02-55019510e0a5

Now retrying with more Kafka partitions and rebased on Petros' PR to improve Pg parallelism.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• 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).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[antiguru]

Seems fine!