[ENH] Add Friedland datasets to chainladder.load_sample().

I've added the Friedland CSVs to the package in the past, but as far as I know I don't think they are accessible via chainladder.load_sample(). This ticket is to remind myself to integrate them.

Also, we'll need to update the [sample data page](https://chainladder-python.readthedocs.io/en/latest/library/sample_data.html#sample-dataset).