Add `NestedFrame.to_pandas` for NestedFrame->DataFrame

[dougbrn]

Realized it we didn't have a ticket for this, but have heard a few times that users would like a NestedFrame.to_pandas option. Even though pd.DataFrame(NestedFrame) is a nice option here, I do think it would be nice to have to_pandas as an intuitive alternative.

[dougbrn]

This already exists! #257

Just making a small follow-up PR to give it a doc example and add it to our API reference registry