docs: add v3 migration guide #4834

Job	Run time
Build	3m 23s
Test example js-libp2p-example-connection-encryption	1m 39s
Test example js-libp2p-example-browser-pubsub	2m 32s
Test example js-libp2p-example-chat	5m 8s
Test example js-libp2p-example-circuit-relay	5m 6s
Test example js-libp2p-example-discovery-mechanisms	4m 51s
Test example js-libp2p-example-pnet	1m 42s
Test example js-libp2p-example-peer-and-content-routing	1m 13s
Test example js-libp2p-example-custom-protocols	1m 33s
Test example js-libp2p-example-delegated-routing	3m 42s
Test example js-libp2p-example-webrtc-private-to-private	3m 24s
Test example js-libp2p-example-protocol-and-stream-muxing	1m 53s
Test example js-libp2p-example-transports	2m 42s
Test example js-libp2p-example-pubsub	2m 55s
	41m 43s

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