Add detailed docs into module README.md files

[mogul]

Story

As a developer (or consumer!) of these Terraform modules,
I would like to see details for a given module,
So that I can understand what it deploys and how it works.

Solution

• Create a README.md in each module, move the top-level README's usage documentation to them, and link to the new location from a summary in the main README
• Add a Mermaid deployment diagram, similar to this one

Acceptance Criteria

• One criteria
• Another criteria