Add details / section about adding externalContracts

[technophile-04]

Once we merge scaffold-eth/scaffold-eth-2#592, it would be really helpful if we add section / page on how to add / interact with external contract maybe we add it #25 section

[carletex]

Another option could be to add it inside the «🛠 Interacting with your Smart Contracts
».

We already have:

If you need to interact with external contracts, you can use wagmi directly, or add external contract data to your deployedContracts.ts file.

Maybe we can just tweak that.