Add link to ARIA WG deep-dive discussion. (#1446)

Author: dbaron

site/src/pages/components/menu.explainer.mdx

@@ -850,3 +850,6 @@ This section links to all of the relevant discussions and [issues](https://githu
 - [[menu] support :open pseudo-class on `<menuitem>`s that open submenus](https://github.com/openui/open-ui/issues/1363)
 - [[css-ui] Add or reuse pseudo-element for openable-submenu indicator (triangle)](https://github.com/w3c/csswg-drafts/issues/13430)
   - This issue has the discussion that chose the name of the `::expand-icon` pseudo-element.
+
+### ARIA
+- [Draft menu-element accessibility proposal](https://github.com/w3c/aria/issues/2782)