Add margin below example headings for better spacing (resolves #975)

Author: arkia09

styles/global.scss

@@ -230,7 +230,7 @@ section,
   h2,
   .text-h2 {
     margin-top: var(--gutter-md);
-    margin-bottom: 0px;
+    margin-bottom: var(--spacing-md);
   }
 
   h2,