docs: Update `GETTING_STARTED.md` docs to give users default configs

[maschad]

We a user lands on the https://github.com/libp2p/js-libp2p/blob/master/doc/GETTING_STARTED.md page, they should primarily see a defaultConfig that would allow them to get started on most networking setups. Here's an example

import { createLibp2p } from 'libp2p'
import { webSockets } from '@libp2p/websockets'
import { noise } from '@chainsafe/libp2p-noise'
import { mplex } from '@libp2p/mplex'
import { yamux } from '@chainsafe/libp2p-yamux',

import { bootstrap } from '@libp2p/bootstrap'

// Known peers addresses
const bootstrapMultiaddrs = [
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN'
]

const node = await createLibp2p({
  transports: [webSockets()],
  connectionEncryption: [noise()],
  streamMuxers: [yamux(), mplex()],
  peerDiscovery: [
    bootstrap({
      list: bootstrapMultiaddrs, // provide array of multiaddrs
    })
  ]
})

node.addEventListener('peer:discovery', (evt) => {
  console.log('Discovered %s', evt.detail.id.toString()) // Log discovered peer
})

node.addEventListener('peer:connect', (evt) => {
  console.log('Connected to %s', evt.detail.remotePeer.toString()) // Log connected peer
})

The user can then go through the remaining sections to learn more about peer discovery, connection encryption, transports, pubsub, stream muxers etc.