Ability to modify Folders' pages#402

Assuming there’s a folder that has the following structure

Shapes
+- Circle
+- Square
+- Triangle

It’d be quite great if we could modify Shapes’ page by using a placeholder document, this one can be done in a similar way to python

Shapes
+- init
+- Circle
+- Square
+- Triangle

The key word init or document would allow in this case, to modify the page that displays first, as an override to the default Folder page