chore: rendezvous example

[vasco-santos]

This PR adds the rendezvous usage example.

Needs:

• [ ] https://github.com/libp2p/js-libp2p-rendezvous/pull/6
• [ ] #856

Other notes:

• In this PR, or as a follow up, we should restructure the discovery-mechanisms examples folder. Currently example 1 is bootstrap, 2 mdns and 3 pubsub discovery, while rendezvous was created in a new folder. I think we should create a folder per type of discovery and partially move the Readme content within each context