Document the interfaces for proving backend integration

[Savio-Sou]

Problem

Since the monorepo'ing effort started, the Noir codebase is now larger than it ever has been.

It could be tricky and unintuitive for proving backend developers to find the correct code interfaces for how they could integrate their proving backends with Noir.

Happy Case

Add a "Backend Integration" page in the Noir docs describing the interfaces for proving backend integration, perhaps with additional code examples of how the default barretenberg backend is integrated as well.

Alternatives Considered

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response