Add example on how to use onchain signer with viem/wagmi

For the onchain signer there are currently only examples with `ethers` in the docs and in the example/demo-voting/demo-quiz.

At ethglobal bangkok, some hackers struggled to implement the onchain signer with `viem/wagmi`. Additional documentation for `viem/wagmi`would  help the devex, especially on hackathons.