website/style: fix code block bottom margin (open-policy-agent#4526)

The text following regular code blocks was too close. Now, the bottom
margin is the same was used with live-blocks, 1.5rem (24px).

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

docs/website/assets/sass/docs.sass

@@ -126,3 +126,6 @@ $panel-header-logo-width: 85%
 .dropdown-menu .dropdown-content
   max-height: 55vh
   overflow-y: auto
+
+.highlight
+  margin-bottom: 1.5rem // matches what live-blocks does