Link equations to functions in docs

[elisachisari]

A suggestion I received: It would be good to make it clear which functions correspond to which equations in the docs. For example, can we give equations for each function here? https://ccl.readthedocs.io/en/latest/api/pyccl.correlations.html

[damonge]

Yeah, I started doing this with the halo model, but it's far from homogeneous. Having this would be great.