feat: adding scenario navigation path

[akaboshinit]

Enhancements to PlaybookGallery:

• Added new properties initialRoute, lightTheme, darkTheme, and themeMode to the PlaybookGallery class in packages/playbook_ui/lib/src/playbook_gallery.dart.
• Updated the PlaybookGalleryState class to use the new properties and support named routes for scenarios. [1] [2] [3]

Modifications to ScenarioContainer:

• Added navigationPath property to ScenarioContainer and updated the onTap method to use named routes instead of MaterialPageRoute. [1] [2]

Other Changes:

• Removed unnecessary MaterialApp wrapper in examples/generator_app/lib/main_catalog.dart and examples/simple_app/lib/main_catalog.dart, and moved theme settings to PlaybookGallery. [1] [2] [3]

[akaboshinit]

TODO: fix Docs