(core) Add tip for "Add New" button

Summary:
Adds a new tip for the doc menu's Add New button. The tip is
shown only when the current user is an editor or owner, and
the site is non-empty. The presence of welcome videos or
popups will also cause the tip to not be shown; it will instead
be shown the next time the doc menu is visited.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3757

app/common/Prefs.ts

@@ -77,6 +77,7 @@ export const BehavioralPrompt = StringUnion(
   'pageWidgetPicker',
   'pageWidgetPickerSelectBy',
   'editCardLayout',
+  'addNew',
 );
 export type BehavioralPrompt = typeof BehavioralPrompt.type;