Hide children endpoints under collapsible top-level endpoints

[deiteris]

Currently, menu navigation shows the entire collection of endpoints which becomes very distracting when working with deep nested API structures and long paths (it's much easier to just find an endpoint in summary than in menu at this point).
In previous versions, children endpoints were hidden under collapsible top-level endpoints which could be expanded when necessary, and that probably will be much easier if menu navigation is structured the same way.