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

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`.