Add documentation describing design of MRI-NUFFT

[paquiteau]

MRI-NUFFT documentation is great, but it lack a document describing the common interfaces for the operators (basically describing the bare minimum API that makes a MRI-NUFFT operator), as well as the expected data format.

This is also the case for the trajectory generation (where we are always in a (Nc,Ns,d) setup).

This would help the community to include new backends/trajectories in MRI-NUFFT, and the other way around, helping the integration of MRI-NUFFT in existing and future tools.