Update docs for kafka default privatelink option for red panda

[jubrad]

Motivation

Adds documentation for https://github.com/MaterializeInc/materialize/issues/24712

Currently this is behind an LD flag so it may be useful to add some private preview notes. I'm also not really sure about the layout, locations I added this documentation, or the level of details for the redpanda.md. Any guidance welcome.

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: