Add explanation section on how we compute radial integrals

[PicoCentauri]

With the implementation of #298 the spliner classes can now handle all combinations of radial basis functions and atomic densities we offer 🎉. And, there is already some technical documentation available. On top some mathematical background explanation would be very useful. i.e. We use a semi-analytical version when we have GaussianDensities etc.

@kvhuguenin already wrote a very good tex document on overleaf which we basically "only" have to cleanup and port to rst.