Add render hooks for codeblocks, headings, and images

layouts/_default/_markup/render-codeblock.html

@@ -0,0 +1,4 @@
+{{- $highlighted := transform.HighlightCodeBlock . -}}
+<pre class="codeblock">
+{{- $highlighted.Inner -}}
+</pre>

layouts/_default/_markup/render-heading.html

@@ -0,0 +1,3 @@
+<h{{ .Level }} id="{{ .Anchor }}" class="heading">
+{{ .Text | safeHTML }} <a class="heading__anchor" href="#{{ .Anchor }}">#</a>
+</h{{ .Level }}>

layouts/_default/_markup/render-image.html

@@ -0,0 +1,15 @@
+{{- if .IsBlock }}
+  <figure id="figure-{{ .Ordinal }}">
+    <img src="{{ .Destination | safeURL }}"
+      {{- with .Text }} alt="{{ . }}"{{ end -}}
+    >
+    {{- with .Title }}
+    <figcaption>{{ . }}</figcaption>
+    {{- end }}
+  </figure>
+{{ else -}}
+  <img src="{{ .Destination | safeURL }}"
+    {{- with .Text }} alt="{{ . }}"{{ end -}}
+    {{- with .Title }} title="{{ . }}"{{ end -}}
+  >
+{{- end }}