Document toy template fit example

[alexander-held]

The toy example for cabinetry contained in example.py should be documented in a way that allows other frameworks to process the same files and compare results. The inputs are created via util/create_histograms.py.

If this example turns out to be useful to projects other than cabinetry, it might also make sense to put it into a separate repository. An expanded setup could allow for an interesting comparison of the fit model implementation via different frameworks.