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/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),
+	})
+}