This PR:

• Removes support for NotFound fragment - it cannot be rendered using NavigationManager.NotFound(). UPDATE DOCS
• Fixes issues with raising NotFound event after async operations.
• Adds support for raising NotFound event without blazor default Router component.

Implementation description

Moves handling 404 signal from endpoint renderer to the endpoint invoker.
Communicates the the renderer that Router handled rendering of NotFoundPage by setting the Path in NotFoundEventArgs. If that property is not set, router treats the request as not handled, cleans the response and avoids flushing in order to allow re-execution middleware act.

Fixes #62367, Fixes #62469