Add UnderlineNav draft component to docs (#2269)

Author: pksjce

docs/src/@primer/gatsby-theme-doctocat/nav.yml

@@ -161,6 +161,8 @@
       url: /drafts/MarkdownEditor
     - title: MarkdownViewer
       url: /drafts/MarkdownViewer
+    - title: UnderlineNav v2
+      url: /drafts/UnderlineNav2
 - title: Deprecated
   children:
     - title: ActionList (legacy)