TutorialBuilder: Make tutorial page folders more modular

[FunctionPoint]

The current setup in the ./Pages folder is:

• The file -Index.json contains the whole page tree.
• The folder structure must mirror the json file.
• The root page of a folder is in the parent folder

Desired new situtuation:

• The file index.json in any folder contains only the file order of files in that folder (not subfolder).
• The folder names indicate the hieracy.
• The tutorial page of a folder is called index.html in its subfolder.
  It is not listen in the file index.json

This way, changes within a folder can be made locally,
updating the file index.json file locally when needed.
And folders can be moved around in the hierarchy without breaking anyting.