Reference the layer system in the docs

[lucasriondel]

Documentation

Relevant Radix Component(s)

Every component that uses a portal (Dialog, Select, Popover, etc).

According to this conversation I had on the primitives repo with @benoitgrelard , I think something should be said concerning the layer system you guys created. I think it's really clever and it would have saved me a lot of time if I had knew about it.

Maybe a simple info card at the top of each component that uses a portal that would redirect to a page explaining how to use the layer system properly ?