Fix ui docs showing up in main docs site (#2787)

Quick fix for some 404 links on the docs site. UI documentation has been
moved to storybook in
#2597, but links to these
pages in the docs sidebar wasn't removed in that PR.

Author: emma-sg

frontend/docs/mkdocs.yml

@@ -99,9 +99,6 @@ nav:
         - develop/ui/components.md
         - develop/ui/storybook.md
         - develop/localization.md
-        - Design:
-          - develop/ui/design-action-menus.md
-          - develop/ui/design-status-icons.md
 
   - API Reference: !ENV [ API_DOCS_URL, "/api/" ]
 
@@ -148,4 +145,4 @@ plugins:
   - search
   - redirects:
       redirect_maps:
-        "user-guide/collections.md": "user-guide/collection.md"
+        "user-guide/collections.md": "user-guide/collection.md"