[WIP] Migration guide for providers to implement V2 primitives

[ElePT]

This is a proposal for content to address #2469. After a brief discussion with @jyu00 we agreed we should focus this migration guide on showing how to implement new primitives for providers. However, I have not been involved in all discussions regarding the vision for this migration (and conversations around backend.run tend to raise a lot of different opinions) so I would appreciate confirmation that the content proposed fits the direction we want to show.

The page is currently in a pretty raw state, missing formatting, API ref links, a review on the titles, capitalization, etc, but I wanted to open a PR as fast as possible to ask for opinions on including the backend primitive wrappers as a tool for provider migration. This idea came from inspecting the only instance I know of external provider beyond the immediate product sphere, which is https://github.com/qiskit-community/qiskit-aqt-provider/tree/master, and subclasses backend primitives to expose their own primitive implementations. Following this, I decided to structure the page as follows:

• motivation: @jyu00 mentioned this was often requested by users, I tried to keep it short and touch on the usefulness of the abstraction level of the primitives
• Backend primitives as a migration alternative for providers that already implemented backend.run (following the AQT example)
• Custom primitives as an alternative for providers that want it, or didn't implement backend.run (following the StatevectorSampler and StatevectorEstimator) examples

Please let me know if you have any other take on how the content should look like @javabster, @jyu00.

[ElePT]

Superseded by #2496