Update top level provider documentation for BackendV2 (#7323)

* Update top level provider documentation for BackendV2

As a follow-on to #5885 this commit updates the top level
qiskit.providers module documentation for BackendV2. This primarily
consists of two things, updating the writing a provider section to use
BackendV2 instead of BackendV1 and adding a migration section on user
facing changes between BackendV1 and BackendV2.

* Apply suggestions from code review

Co-authored-by: Luciano Bello <[email protected]>

* Update qiskit-aqt example wording

* Fix lint

Co-authored-by: Luciano Bello <[email protected]>