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

media/docshelper.go

@@ -0,0 +1,17 @@
+package media
+
+import (
+	"github.com/spf13/hugo/docshelper"
+)
+
+// This is is just some helpers used to create some JSON used in the Hugo docs.
+func init() {
+	docsProvider := func() map[string]interface{} {
+		docs := make(map[string]interface{})
+
+		docs["types"] = DefaultTypes
+		return docs
+	}
+
+	docshelper.AddDocProvider("media", docsProvider)
+}