Add subsection 6 Reverse-Proxies to explain URI embedding options (take from RFC 8075)

It would be good to mention for the reverse-proxy it may use (preferably?) the URI embedding options of RFC 8075. This can also be made discoverable (including mapping) as defined in that RFC; so then we need to define a resource type (rt) like core.hc but then for coap-coap reverse proxy.