Remove custom example plugin. (#25784)

_includes/example.html

@@ -0,0 +1,23 @@
+{%- comment -%}
+Usage: {% include example.html content=markup %},
+where content is a capture with the HTML content
+id - null (default)
+class - "bd-example" (default)
+optional: hide_preview - disabled (default)
+optional: hide_markup - disabled (default)
+{%- endcomment -%}
+
+{%- assign preview_id = include.id -%}
+{%- assign preview_class = include.class -%}
+
+{%- if include.hide_preview == null -%}
+<div{% if preview_id %} id="{{ preview_id }}"{% endif %} class="bd-example{% if preview_class %} {{ preview_class }}{% endif %}">
+  {{- include.content -}}
+</div>
+{%- endif -%}
+
+{%- if include.hide_markup == null -%}
+  {%- highlight html -%}
+    {{- include.content | replace: 'data-src="holder.js', 'src="...' -}}
+  {%- endhighlight -%}
+{%- endif -%}