Add mermaid diagram rendering to docs site

[kmcginnes]

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 Migrate documentation to Starlight (Astro) #1754