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

docshelper/docs.go

@@ -0,0 +1,32 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package docshelper provides some helpers for the Hugo documentation, and
+// is of limited interest for the general Hugo user.
+package docshelper
+
+import (
+	"encoding/json"
+)
+
+var DocProviders = make(map[string]DocProvider)
+
+func AddDocProvider(name string, provider DocProvider) {
+	DocProviders[name] = provider
+}
+
+type DocProvider func() map[string]interface{}
+
+func (d DocProvider) MarshalJSON() ([]byte, error) {
+	return json.MarshalIndent(d(), "", "  ")
+}