performance: use hugo render hook for Mermaid

This switches Mermaid to use hugo's Markdown
render hooks [1] instead of manipulating the string at runtime, which
should improve performance by a bit.

[1]: https://gohugo.io/templates/render-hooks/#render-hooks-for-code-blocks

Signed-off-by: Stephan Lachnit <stephanlachnit@debian.org>

assets/js/mermaid.js

@@ -2,10 +2,9 @@
 {{ if .enable }}
 (function($) {
     var needMermaid = false;
-    $('.language-mermaid').parent().replaceWith(function() {
+    if ($('.mermaid').length > 0) {
         needMermaid = true;
-        return $('<pre class="mermaid">').text($(this).text());
-    });
+    };
 
     if (!needMermaid)  {
         mermaid.initialize({startOnLoad: false});

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

@@ -0,0 +1,3 @@
+<div class="mermaid">
+  {{- .Inner | safeHTML }}
+</div>