Simplify page hierarchies

Too many #youhavetoknow details to make Wiki page hierarchies work. There's a help text, but it's far from intuitive and doesn't provide full info.

1. Know the page slug of the parent page, look it up in the URL. Example: setup
2. Create a new subpage with the slug including the parent in typical / folder hierarchy syntax. Example: setup/new-subhierarchy
3. Make sure the page title is the same, including the parent hierarchy. Example: Setup/New Subhierarchy. Don’t ever dare to use spaces around the hierarchy part. You can rename it once the page is created.