clarify that Moffat profiles do not support autodiff with respect to beta

[beckermr]

Right now when we flatten Moffat profiles to pytrees, we include beta in the traceable parameters. We should instead add it to the metadata.

[beckermr]

We should also update the docs to reflect that we cannot compute gradients with respect to beta.

[beckermr]

xref: #199

It turns out it is only relevant for certain cases in either real- or Fourier-space.