Add support for Mermaid diagrams

Mermaid diagrams (https://mermaid.js.org/) have become a common thing
ever since GitHub introduced support for them in GitHub-flavored
Markdown codeblocks.

With this here commit, we can now also use those diagrams on
git-scm.com. The idea is to use the new `mermaid` shortcode, like this:

  {{< mermaid >}}
  graph LR
    A --> B
  {{< /mermaid >}}

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

layouts/_default/baseof.html

@@ -191,5 +191,11 @@ <h1 data-pagefind-meta="title">About{{ if (isset .Params "subtitle") }} - {{ .Pa
   {{ end }}
 
 </body>
+{{- 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 });
+  </script>
+{{ end -}}
 </html>
 {{ end }}

layouts/shortcodes/mermaid.html

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