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)
+}

media/mediaType.go

@@ -14,6 +14,7 @@
 package media
 
 import (
+	"encoding/json"
 	"fmt"
 	"sort"
 	"strings"
@@ -177,3 +178,16 @@ func DecodeTypes(maps ...map[string]interface{}) (Types, error) {
 
 	return m, nil
 }
+
+func (t Type) MarshalJSON() ([]byte, error) {
+	type Alias Type
+	return json.Marshal(&struct {
+		Type   string
+		String string
+		Alias
+	}{
+		Type:   t.Type(),
+		String: t.String(),
+		Alias:  (Alias)(t),
+	})
+}