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

@@ -0,0 +1,18 @@
+{{ $width := .Attributes.width }}
+{{ $height := .Attributes.height }}
+{{ $class := .Attributes.class | default "" }}
+<div class="goat svg-container {{ $class }}">
+  {{ with diagrams.Goat .Code }}
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      font-family="Menlo,Lucida Console,monospace"
+      {{ if or $width $height }}
+        {{ with $width }}width="{{ . }}"{{ end }}
+        {{ with $height }}height="{{ . }}"{{ end }}
+      {{ else }}
+        viewBox="0 0 {{ .Width }} {{ .Height }}"
+      {{ end }}>
+      {{ .Body }}
+    </svg>
+  {{ end }}
+</div>

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

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