add shadow for card and callout

docs/50_Components/Callout.md

@@ -24,6 +24,7 @@ Key      | Description
 ---      | ---
 class    | optional: additional css classes
 type     | one of [none]\|danger\|dark\|info\|primary\|secondary\|success\|warning
+shadow   | size of shadow; one of [none] (=default: between small and regular)|none|small|regular|large
 
 Content: