Allow for usage of (group) advanced routing

[HighPriest]

My use case is to get different layouts, without nesting pages: e.g. this can give me docs/ in water layout and docs/admin in fire layout

docs/(water)/+page.md
docs/(water)/+layout.svelte
docs/(fire)/+layout.svelte
docs/(fire)/admin/+page.md

Currently, if we use this type of routing, I have noticed at least the nav-side-bar contents and links get broken, but probably some other things will too.

[CaptainCodeman]

I've looked at the same thing, to allow hosting examples in iframes (so an empty root layout, with the layout for docs within a (docs) group). Haven't got to the stage of making it work yet.