The MSDN library online has been refreshed, and you can find all the new documentation for the WPF Designer for Visual Studio (formerly known as "Cider") here. You might want to start with the overview topic, or if you want to start autoring your own custom design time, try the extensibility portal topic.

Here's some random artwork from the docs to give you a taste.

 

Design view features in the WPF Designer

Design View Features

 

 

XAML view features in the WPF Designer

XAML View Features