Document how to test contract callbacks and contract receipts

[akorchyn]

The user can provide promise_results to the testing_env to test the contract callbacks.

Also, the user can get the receipts from the contract execution using get_created_receipts

Currently, the documentation lacks examples for that

[pheobeayo]

Could I take on this issue?

[akorchyn]

Yeah, sure.

[pheobeayo]

@akorchyn, Kindly assign to me