Document techniques for debugging templates.

This information was previously scattered around in the forums and
mailing list.  Add it to the official docs to make things easier for new
users.

Fixes #1167

docs/content/templates/404.md

@@ -8,6 +8,7 @@ menu:
     parent: layout
 next: /taxonomies/overview
 notoc: true
+next: /templates/debugging
 prev: /templates/sitemap
 title: 404.html Templates
 weight: 100

docs/content/templates/debugging.md

@@ -0,0 +1,55 @@
+---
+aliases:
+- /doc/debugging/
+- /layout/debugging/
+date: 2015-05-22
+linktitle: Debugging
+menu:
+  main:
+    parent: layout
+prev: /templates/404
+title: Template Debugging
+weight: 110
+---
+
+
+# Template Debugging
+
+Here are some snippets you can add to your template to answer some common questions.
+These snippets use the `printf` function available in all Go templates.  This function is
+an alias to the Go function, [fmt.Printf](http://golang.org/pkg/fmt/).
+
+### What type of page is this?
+
+Does Hugo consider this page to be a "Node" or a "Page"? (Put this snippet at
+the top level of your template. Don't use it inside of a `range` loop.)
+
+    {{ printf "%T" . }}
+
+
+### What variables are available in this context?
+
+You can use the template syntax, `$.`, to get the top-level template context
+from anywhere in your template.  This will print out all the values under, `.Site`.
+
+    {{ printf "%#v" $.Site }}
+
+This will print out the value of `.Permalink`, which is available on both Nodes
+and Pages.
+
+    {{ printf "%#v" .Permalink }}
+
+This will print out a list of all the variables scoped to the current context
+(a.k.a. The dot, "`.`").
+
+    {{ printf "%#v" . }}
+
+When writing a [Homepage](/templates/homepage), what does one of the pages
+you're looping through look like?
+
+```
+{{ range .Data.Pages }}
+    {{/* The context, ".", is now a Page */}}
+    {{ printf "%#v" . }}
+{{ end }}
+```