Add support for Mermaid diagrams

Per instructions from Hugo:
https://gohugo.io/content-management/diagrams/#mermaid-diagrams

The styling could probably be updated to be more in line with the rest
of the docs, but this already looks better to me than the goat ASCII
diagrams Hugo supports out of the box.

Signed-off-by: Nic Cope <nicc@rk0n.org>

themes/geekboot/layouts/_default/_markup/render-codeblock-mermaid.html

@@ -0,0 +1,4 @@
+<pre class="mermaid">
+  {{- .Inner | safeHTML }}
+</pre>
+{{ .Page.Store.Set "hasMermaid" true }}

themes/geekboot/layouts/partials/single-list.html

@@ -75,3 +75,10 @@
         </div>
     </main>
   </div>
+
+  {{ if .Page.Store.Get "hasMermaid" }}
+  <script type="module">
+    import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.esm.min.mjs';
+    mermaid.initialize({ startOnLoad: true });
+  </script>
+{{ end }}