maxtar.github.io/themes/hugo-geekdoc/layouts/_default/_markup/render-codeblock-mermaid.html

<!-- prettier-ignore-start -->
{{ if not (.Page.Scratch.Get "mermaid") }}
  <!-- Include mermaid only first time -->
  <script defer src="{{ index (index .Page.Site.Data.assets "mermaid.js") "src" | relURL }}"></script>
  {{ .Page.Scratch.Set "mermaid" true }}
{{ end }}
<!-- prettier-ignore-end -->

<pre class="gdoc-mermaid mermaid text-center">
  {{- .Inner -}}
</pre>