readded hugo-book theme files.

themes/hugo-book/layouts/_markup/render-codeblock-mermaid.html

@@ -0,0 +1,10 @@
+{{- /* remove whitespaces */ -}}
+<pre class="mermaid">{{ .Inner | htmlEscape | safeHTML -}}</pre>
+{{- if not (.Page.Store.Get "mermaid") -}}
+  {{- /* Include mermaid only first time */ -}}
+  <script src="{{ "mermaid.min.js" | relURL }}"></script>
+  {{- with resources.Get "mermaid.json" | minify -}}
+    <script>mermaid.initialize({{ .Content | safeJS }})</script>
+  {{- end -}}
+  {{- .Page.Store.Set "mermaid" true -}}
+{{- end -}}