Document how to get a contract's balance via Sandbox API

[anton-trunov]

It can be done like so:

const contractBalance = (await blockchain.getContract(contract.address)).balance;

There is no need to add a getter to your contract to test how the contract's balance is changing:

get fun balance(): Int { return myBalance() }

P.S.: (novusnota) Thinking of a separate page for the Blueprint & Sandbox. In the Reference → Tools / Third-party.