all: Document the Output Formats feature

This commit also adds a new command, docshelper, with some utility funcs that adds a JSON datafiles to /docs/data that would be a pain to create and maintain by hand.

Fixes #3242

docs/layouts/shortcodes/datatable.html

@@ -0,0 +1,23 @@
+{{ $package := (index .Params 0) }}
+{{ $listname := (index .Params 1) }}
+{{ $list := (index (index .Site.Data.docs $package) $listname) }}
+{{ $fields := after 2 .Params }}
+
+<table class="table table-bordered">
+	<tr>
+		{{ range $fields }}
+		<th>{{ . }}</th>
+		{{ end }}
+	</tr>
+	{{ range $list }}
+	<tr>
+		{{ range $k, $v := . }}
+		{{ $.Scratch.Set $k $v }}
+		{{ end }}
+		{{ range $fields }}
+			<td>{{ $.Scratch.Get . }}</td>
+		{{ end }}		
+	</tr>
+	{{ end }}
+</table>
+