Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

docs/_vendor/github.com/gohugoio/gohugoioTheme/layouts/_default/baseof.html

@@ -66,6 +66,14 @@
 
   {{ block "footer" . }}{{ partialCached "site-footer.html" . }}{{ end }}
 
+  {{ if .Page.Store.Get "hasMermaid" }}
+  <script src="https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js"></script>
+  <script>
+    mermaid.initialize({ startOnLoad: true });
+  </script>
+{{ end }}
+
+
 </body>
 
 </html>