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

commands/hugo.go

@@ -197,6 +197,7 @@ func AddCommands() {
 	genCmd.AddCommand(genautocompleteCmd)
 	genCmd.AddCommand(gendocCmd)
 	genCmd.AddCommand(genmanCmd)
+	genCmd.AddCommand(createGenDocsHelper().cmd)
 }
 
 // initHugoBuilderFlags initializes all common flags, typically used by the