Should cover the basics of the content routing interface and how it's used by other parts of libp2p (e.g. rendezvous for AutoRelay). Should mention that the primary implementation is via kad-dht, but others implementations are possible.

This should be complementary to #13, which covers Peer Routing. Since the content routing interface is distinct from peer routing (even if both are often backed by the same DHT implementation), we should discuss it separately and link between the docs where it makes sense.