Add examples with different frontend/backend combinations

[rmlarose]

If the library works as intended, different frontends/backends should really work together seamlessly, but it will still be good to highlight this functionality by showing explicit examples. There are in principle four different combinations as of now since we support two frontends and two backends.

• Cirq frontend with Qiskit backend.
• Qiskit frontend with Qiskit backend.
• Cirq frontend with Cirq backend.
• Qiskit frontend with Cirq backend.

Self-assigned but collaborators welcomed.

[willzeng]

As this is still in progress I propose we move this to milestone 2? Any opposed?

[rmlarose]

Pull #140 completes Qiskit/Cirq frontends on a Qiskit backend. I think Qiskit/Cirq frontends on Cirq backends is overkill given the other docs. What do you think @willzeng? We can close this if you agree.

[willzeng]

agreed. You note that any front end / back end combo can work in the docs