change music store example to community toolkit

[AnastassiaP]

change reactive framework mentions to CommunityToolkit.Mvvm

[thevortexcloud]

Why? ReactiveUI is still supported and very widely used. It would make more sense to make a separate version of the guide for using Community MVVM. But it ends up being more annoying to maintain.

[MikeCodesDotNET]

Why? ReactiveUI is still supported and very widely used. It would make more sense to make a separate version of the guide for using Community MVVM. But it ends up being more annoying to maintain.

We've had feedback from multiple people that our docs jumping straight into ReactiveUI makes it harder to learn.

We're going to be removing ReactiveUI as the default option from the docs and samples. It adds another level of complexity for those learning Avalonia, which isn't helpful.

Our docs should be opinionated, and focused on ease of use.