Port to using Kirigami.PageRouter for navigation

Needs frameworks/kirigami!30 (merged)

The PageRouter is designed for exactly what Navigation does in Discover: arbitrary navigation through routes. By porting to a shared component instead of rolling a custom solution, potential contributors are able to refer to both other applications and documentation on api.kde.org instead of needing to learn a custom Discover-specific solution for navigation. This also reduces the potential of bitrot as the PageRouter is shared between applications and is actively maintained as a part of Kirigami's public API instead of as part of the internals of a single application.

Additionally, the PageRouter has intelligent caching allowing for less "freeze" and more "go" when navigating through Discover.