Improve the echo include

And add a dump of `page` to the editing/debugging page.

Author: ctrueden

_includes/echo

@@ -1,4 +1,6 @@
 {%- comment -%}
 This include just echoes its arguments, for debugging/learning purposes.
 {%- endcomment -%}
-<p>{{include}}</p>
+```
+{{include}}
+```

_pages/editing/debugging.md

@@ -16,3 +16,12 @@ Note that liquid allows the passing of [arbitrary arguments](https://jekyllrb.co
 
 {% highlight liquid %}{% raw %}{% include echo content="This is the content argument" bracket="what if I pass {}?" gorilla="Why do we have a gorilla argument?" %}{% endraw %}{% endhighlight %}
 {% include echo content="This is the content argument" bracket="what if I pass {}?" gorilla="Why do we have a gorilla argument?" %}
+
+### What's in a `page`?
+
+{% include notice icon="tech" content="Ever wonder what all is part of the `page` data structure? Let's find out! 
+
+(You can also do this with the `site` data structure, but be warned: the output will include every page body across the whole site!)" %}
+
+{% highlight liquid %}{% raw %}{% include echo p=page %}{% endraw %}{% endhighlight %}
+{% include echo p=page %}