Setup doctests

[oyeb]

doctests will help us catch any stale examples in the documentation, in case the examples break due to changes in code.

The following need to be resolved for doctests to work:

1. The outputs also have to be written down in the documentation

• Typical response objects are huge (will take up a lot of space).
• Responses are unique and can't be tested against.

2. Our examples invoke calls on the :payment_worker but there's no gateway configuration in the app.
3. We might be able to mock the remotes during the doctests too, but I'm not sure how. Mocking is important because all doctests will fail otherwise, because the remote responses are unique each time and we can't write the responses down in the docs.