Refactor: Simplify navigation logic in ProgressIndicatorNavGraph

This commit simplifies the navigation logic within `ProgressIndicatorNavGraph.kt`. The `onNavigateToExample` lambda in `ProgressIndicatorListingScreen` now directly adds the provided route to the `backStack`, removing the need for a `when` statement to handle different `ProgressIndicatorRoutes`.

Additionally, `ProgressIndicatorRoutes` now directly inherits from `NavKey`, removing redundant `NavKey` inheritance from its nested objects. The unused `ButtonGroupRoutes` object within `AppRoutes` has also been removed.