Convert architectural overview diagrams from JPG images to mermaid

[jasonmadigan]

Summary

The architectural overview page uses 7 embedded JPG images for its diagrams. These should be converted to mermaid format for maintainability and consistency.

Motivation

• JPG diagrams go stale — they require image editing tools to update, so they tend to drift from reality
• Mermaid diagrams are text-based and live in the markdown source, making them easy to update alongside code changes
• Other docs pages already use mermaid successfully (e.g. the egress credential injection guide)
• Mermaid renders natively in MkDocs Material and looks consistent with the rest of the site

Diagrams to convert

1. Highest-level architecture (images/highest-level.jpg)
2. Control plane overview (images/control-plane-overview.jpg)
3. Data plane overview (images/data-plane-overview.jpg)
4. Single cluster layout (images/single-cluster-layout.jpg)
5. Multi-cluster layout (images/multi-cluster-layout.jpg)
6. Single cluster observability (images/arch_observability_1.jpg)
7. Multi-cluster observability (images/arch_observability_2.jpg)

Acceptance criteria

• All 7 diagrams replaced with mermaid equivalents
• Mermaid diagrams accurately represent the same information as the original images
• Original JPG files removed from the repo
• Page renders correctly with mkdocs serve -s

Reviewing a PR for this

• Compare each mermaid diagram against the original JPG to verify accuracy
• Check the rendered output with mkdocs serve -s on the architectural overview page
• Verify no broken image references remain