Add to docs what types are supported out of the box

Feedback from a GBB working with a certain big clothing chain customer.

The customer was trying to return a tuple consisting of a list of custom type and Pandas dataframe. It was not obvious that when an activity function returns, it's actually serialized and deserialized back in the orchestrator function. They were getting error that the tuple didn't implement the `from_json` method .

There is an example in this repo on how to use a custom class as the result, but that's not mentioned anywhere in docs. Would be good to add what types are supported out of the box, i.e. those implementing `to_json` and `from_json`.