New config: Format top folders as "categories"

[xy2z]

A new boolean config setting for rendering top folders as categories or folders.

Folders is how it is now - a tree view
A "categories" rendering example could be: https://docs.gitbook.com/
So if a folder is named "Getting started" all files are rendered below, without an option to show/hide files.

config name could be something like nav_folder_tree (default true)