add document on core client / tower layer behavior

[clux]

we only have lose docs on client, tower setup, layers and it's squirrelled away within the controbutor architecture docs.

It'd be good to document them, better here as an overview without having to force everyone to jump between tower, our scattered layer docs, our configext docs and our layer setup gunk (which is private).

This is a fairly complicated component, but one that can probably be explained in a more helpful way than we currently do.

See kube-rs/kube#1559 (reply in thread) for context.