Improved Simulator documentation

I'm working here on a little tutorial on how to use the Simulator interface to create a new simulation or adapt existing simulation code.

This is building on the example given in https://gitlab.inria.fr/ml_genetics/private/dnadna/-/blob/master/examples/example_simulate_msprime_save_dnadna_npz.ipynb

On one hand, that example might require a little less overhead to understand. On the other hand, implementing the plugin interface hides away a lot of details of writing the DNADNA data format.