Clarify making a multi-page app using Avalonia

[andrewhong04]

I see that it was in the works, but is incomplete.

https://github.com/AvaloniaUI/avalonia-docs/blob/main/docs/guides/development-guides/how-to-implement-multi-page-apps.md

[thevortexcloud]

There is already a guide, it's just ReactiveUI specific.

https://docs.avaloniaui.net/docs/concepts/reactiveui/routing

There is also a non ReactiveUI example here:

https://github.com/AvaloniaUI/Avalonia.Samples/tree/main/src/Avalonia.Samples/Routing