docs: update routing overview doc to add TypedShellRoute example

[AnnaPS]

Hi!
I've read our VGE about Go Router and after doing a POC using that approach (since I've never used TypedGoRoute in projects), I realized that we don't talk about anything related to ShellRoute in that documentation. So I've been playing around with Go_Router and I've discovered that there's TypedShellRoute, which allows you to have nested routes, like for BottomNavigationBar, for example.
I think ShellRoute is common in mobile apps and it might be interesting to include it in that documentation.

Thoughts?

Example

@TypedShellRoute<AppShellRoute>(
  routes: <TypedRoute<RouteData>>[
    TypedGoRoute<FirstPageRoute>(
      path: FirstPageRoute.path,
      routes: [
        TypedGoRoute<DetailPageRoute>(
          path: DetailPageRoute.path,
        ),
      ],
    ),
    TypedGoRoute<SecondPageRoute>(
      path: SecondPageRoute.path,
    ),
  ],
)