content/static: add margin to bottom of last element in nav group

Fixes issue where a single item in a nav group
like Index > Examples looks cluttered.

Change-Id: Icf9103c4f6e7b57339df7187ca593f4ef604367f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260727
Trust: Jamal Carvalho <[email protected]>
Run-TryBot: Jamal Carvalho <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>

Author: jamalc

content/static/css/sidenav.css

@@ -159,6 +159,9 @@
   .DocNav [role='group'] [role='treeitem'] {
     padding-left: 2.5rem;
   }
+  .DocNav ul[role='group'] > :last-child {
+    margin-bottom: 0.375rem;
+  }
   .DocNav [aria-selected='true'] {
     color: var(--gray-1);
     font-weight: 500;