Add examples for every command in docs

[piotmag769]

Currently some of our commands don't have example usages, some of them do.
We should make it consistent.

https://docs.swmansion.com/protostar/docs/cli-reference

To add an example just create a function in a class implementing a command

@property
def example(self) -> Optional[str]:
    return "protostar deploy 0x4221deadbeef123 --network testnet"