Nested Pages in Side Navigation

### Description

It's come up a few times recently to support nested pages in a navigation.

This would mean having headings in the navigation side bar that could be collapsable (or static) to allow easier grouping of pages.

Need some thought about how we support this, and there are then challenges of how this is represented in the sidebar too.

It's likely going to need to be a new config node type

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate