feat(docs): add template preview (#1777)

pkg/notifications/templates/funcs.go

@@ -0,0 +1,27 @@
+package templates
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"text/template"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+)
+
+var Funcs = template.FuncMap{
+	"ToUpper": strings.ToUpper,
+	"ToLower": strings.ToLower,
+	"ToJSON":  toJSON,
+	"Title":   cases.Title(language.AmericanEnglish).String,
+}
+
+func toJSON(v interface{}) string {
+	var bytes []byte
+	var err error
+	if bytes, err = json.MarshalIndent(v, "", "  "); err != nil {
+		return fmt.Sprintf("failed to marshal JSON in notification template: %v", err)
+	}
+	return string(bytes)
+}