Implement the navigation nesting logic

So if I understand this correctly, you want the nesting to happen with the entries and their respective categories (one layer of nesting), right? Or do you want items to be able to contain other sub-items now (infinite nesting)? This would also require a change of how entity contents are created, as well as a new markdown metadata key.

@lrwebks right questions! We should discuss this with @thomas.frobieter later based on our concrete examples.

Let's postpone this, until we need it. Later maybe the grouping by category is enough, but should be a separate issue. And maybe handled in JS, so it can be switched?

For the menu we MIGHT need this feature later to be able to cascade the entries, but not relevant yet.