Listbox Design Pattern: Add link to grouped listbox example (pull #1341)

Co-authored-by: Matt King <[email protected]>

aria-practices.html

@@ -1805,6 +1805,7 @@ <h4>Examples</h4>
           <li><a href="examples/listbox/listbox-scrollable.html">Scrollable Listbox Example</a>: Single-select listbox that scrolls to reveal more options, similar to HTML <code>select</code> with <code>size</code> attribute greater than one.</li>
           <li><a href="examples/listbox/listbox-collapsible.html">Collapsible Dropdown Listbox Example</a>: Single-select collapsible listbox that expands when activated, similar to HTML <code>select</code> with the attribute <code>size=&quot;1&quot;</code>.</li>
           <li><a href="examples/listbox/listbox-rearrangeable.html">Example Listboxes with Rearrangeable Options</a>: Examples of both single-select and multi-select listboxes with accompanying toolbars where options can be added, moved, and removed.</li>
+          <li><a href="examples/listbox/listbox-grouped.html">Listbox Example with Grouped Options</a>: Single-select listbox with grouped options, similar to an HTML <code>select</code> with <code>optgroup</code> children.</li>
         </ul>
       </section>