[core] Define routes statically

[Janpot]

Define the routes statically. In preparation for moving to react-router new API with createBrowserRouter. Which allows for lazy-loading routes. This will be interesting in the long term future to do code splitting between pages, but necessary in the short term to host the editor alongside the application for #3171

Statically means that the whole routing is now defined in:

<Routes>
  <Route path="/signin" element={<SignInPage />} />
  <Route path="/" element={<ToolpadAppLayout />}>
    <Route path="/pages/:pageName" element={<RenderedPage />} />
    <Route path="/pages" element={<DefaultPageNavigation />} />
    <Route path="/" element={<DefaultPageNavigation />} />
    <Route path="*" element={<PageNotFound />} />
  </Route>
</Routes>

In this PR:

• defines the routes statically
• fixes error when no pages are available in the app
• uses outlet to render child routes in app layout

[Janpot]

I'm merging this. Feel free to review in retrospect.

[bharatkashyap]

👍🏻