WinUI3 support clarification - documentation update

[get-flat]

Hi, it seems that, at least for me, it's not always very straightforward to understand if CommunityToolkit is supposed to work together with WinUI3 (vs UWP WinUI2).

https://docs.microsoft.com/pl-pl/windows/communitytoolkit/

It simplifies and demonstrates common developer tasks building UWP apps for Windows 10.

Toolkit sample gallery: are all Controls supposed to work with WinUI3? i.e. ViewExtensions
(if not all controls are supported maybe WinUI 2 vs WinUI 3 gallery separation approach can be adopted)

Having generic projects i.e. MVVM toolkit is a great direction 👍