Add a 20px margin on the sticky contents

Add a margin to the top of the contents list on service manual guides so
it isn't too high up the page when scrolling

app/assets/stylesheets/modules/_sticky.scss

@@ -1,6 +1,6 @@
 .app-sticky-element {
   @include govuk-media-query($from: tablet) {
     position: sticky;
-    top: 0;
+    top: govuk-spacing(4);
   }
 }