Add "real" ggplot examples to `label_*()` and `breaks_*()` functions

[mine-cetinkaya-rundel]

Currently the examples in the docs for label_*() and breaks_*() functions use demo_*() to showcase what various functions do, but they don't exemplify how to use them in the scale_*() layer of a ggplot. It would be good to add a simple, real plotting example as the first example in each of these functions. Then, the demo functions can be left as is to avoid code duplication to showcase various features and for other (testing, etc.) benefits these functions might have.