Return the dummy keys to /examples/mina-zk-app

[alexandra-mara]

Right now we're describing updating keys for mina-zk-app in our documentation and zkApp's readme, but the keys folder was removed from the repo and added to .gitignore some time ago. I suggest we add sample files keys/berkeley.json and keys/user.json, so users would know exactly what is supposed to be in these files. Something like this:

{
  "privateKey": "YOUR_PRIVATE_KEY",
  "publicKey": "YOUR_PUBLIC_KEY"
}

[NickVolynkin]

It's a dangerous practice because, in the end, people will accidentally commit their secrets and push them to GitHub. The best we can do is keep the directory by adding an empty .gitkeep file.