Add an index route too

Closes #12460

Author: timdorr

docs/upgrading/future.md

@@ -57,6 +57,7 @@ Split any multi-segment splat `<Route>` into a parent route with the path and a
   <Route path="/" element={<Home />} />
 -  <Route path="dashboard/*" element={<Dashboard />} />
 +  <Route path="dashboard">
++    <Route index element={<Dashboard />} />
 +    <Route path="*" element={<Dashboard />} />
 +  </Route>
 </Routes>