Add/generate examples for operators

[jsntsay]

Similar to sklearn docs, each operator should have some working examples to demonstrate how to use them

[hirzel]

We could use doctest. That way, the examples would serve a dual purpose of both illustrating intended usage and detecting regressions in the functionality. We would need to set it up so the doctests run as part of our GitHub Actions CI tests. However, another complicating factor is that the docstrings for operators are auto-generated from schemas. So we would want to make sure the flow from schemas to docstrings to doctests to regression tests works.

By the way, a handful of operators already have embedded code examples today:

• ConcatFeatures
• Project
• Hyperopt
• SMAC