add danger callout (#789)

Author: dks333

components/callouts.mdx

@@ -36,6 +36,12 @@ Callouts can be styled as a Note, Warning, Info, Tip, or Check:
 <Check>This brings us a checked status</Check>
 ```
 
+<Danger>This is a danger callout</Danger>
+
+```jsx
+<Danger>This is a danger callout</Danger>
+```
+
 <RequestExample>
 
 ```jsx Callout Example