docs: add v3 migration guide #4836

Job	Run time
Build	3m 11s
Test example js-libp2p-example-chat	3m 50s
Test example js-libp2p-example-browser-pubsub	4m 2s
Test example js-libp2p-example-connection-encryption	1m 29s
Test example js-libp2p-example-circuit-relay	5m 19s
Test example js-libp2p-example-delegated-routing	2m 33s
Test example js-libp2p-example-custom-protocols	1m 54s
Test example js-libp2p-example-discovery-mechanisms	3m 13s
Test example js-libp2p-example-pnet	3m 36s
Test example js-libp2p-example-webrtc-private-to-private	2m 15s
Test example js-libp2p-example-transports	2m 59s
Test example js-libp2p-example-protocol-and-stream-muxing	2m 52s
Test example js-libp2p-example-peer-and-content-routing	1m 59s
Test example js-libp2p-example-pubsub	1m 31s
	40m 43s

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