Add mermaid diagram rendering to docs site

## Description

The `references/architecture.md` page contains a mermaid diagram that renders as a plain code block in Starlight. Starlight does not render mermaid diagrams by default.

## Solution

Add the `rehype-mermaid` plugin (or `@astrojs/starlight-mermaid`) to the Astro config so that ```mermaid``` code blocks render as diagrams.

## Acceptance Criteria

- Mermaid code blocks in documentation render as SVG diagrams
- The docs build still passes
- No additional client-side JavaScript is required (use server-side rendering)

## Related Issues

- Parent #1754