[Generator Concept + Plane Ticket Exercise]: Consider Adding list of common generator-iterators from the Std. Library

[BethanyG]

As we are working through initial feedback for the Plane Tickets exercise and the generators concept, it occurred to me that many students are unaware of places/objects in the stdlib that return/are generator-iterators. We should consider where in the docs to point out some examples (like zip() and various itertools). We might also want to mention generator-expressions in passing in about.md.