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

This request is far too low down the list!