feat: add button design to rte

packages/base/Configuration/RTE/Default.yaml

@@ -12,6 +12,8 @@ editor:
             - { name: "Small", element: "p", attributes: { 'class': 'small' } }
             - { name: "Big Number", element: "span", attributes: { 'class': 'big-number' } }
 
+            - { name: "Button", element: "a", attributes: { 'class': 'btn' } }
+
         toolbarGroups:
             - { name: styles, groups: [ format, styles ] }
             - { name: basicstyles, groups: [ basicstyles ] }