[docs] Add docs for direct usage of backend functions in the app

[CuriousLearner]

We have usage docs: https://github.com/CuriousLearner/django-phone-verify/blob/master/docs/usage.rst which describes on how to use this package in terms of API. I think we can improve this documentation by providing examples on how to import the backend set in settings by calling phone_verify.backends.get_sms_backend.