Add docs section on serialization

[nstarman]

Thanks for the great package! It would be nice to have a docs section that explains / or points to an explanation of how to serialize and unserialize the objects in this package: Bijections, Flows (and their weights), etc.

[danielward27]

Thanks! As the distributions and bijections are just equinox modules, we can serialize/deserialize them using the same method outlined in the equinox documentation. I'll add a note to the FAQ. Let me know if you run into any issues.