Add support for Mermaid diagrams in markdown content

[miklz]

Add Mermaid Diagram Support to the Theme

This PR adds native Mermaid diagram rendering support to markdown content.
It allows users to include diagrams directly in code fences, e.g.:

graph TD
  A[Start] --> B{Is it working?}
  B -->|Yes| C[Ship it!]
  B -->|No| D[Fix it]

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Implementation Details

• Added a new custom renderer:
  layouts/_default/render-codeblock-mermaid.html
  This file wraps Mermaid code blocks in a <pre class="mermaid"> tag and sets a flag (hasMermaid) on the page context.

• Updated baseof.html to conditionally include the Mermaid JavaScript module from the CDN only when needed:

{{ if .Store.Get "hasMermaid" }}
  <script type="module">
    import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.esm.min.mjs';
    mermaid.initialize({
      startOnLoad: true,
      theme: 'neutral',
    });
  </script>
{{ end }}

Benefits

• No external preprocessing or shortcodes required.
• Lightweight and only loaded when a page actually includes Mermaid diagrams.
• Works seamlessly with existing markdown content and Hugo’s markup system.