Document the Design and DesignTools projects and their purpose.

[Rosuavio]

I felt like this was kinda a hidden section of the Repo. It could be good to document the purpose of the Design projects and the DesignTools projects coming from CommunityToolkit/WindowsCommunityToolkit#3318.

[Rosuavio]

I found this very useful for understanding the projects.
https://github.com/microsoft/xaml-designer-extensibility

[Nirmal4G]

Will do.