Improve examples throughout documentation (and remove dependencies?)

[rpruim]

The examples have served in part to check that features work as advertised, but some of them are not very interesting and could be improved.

Also, several of them rely on external packages (likemosaicData), which we might prefer to avoid. (But using external packages can contribute to more interesting examples.)

Perhaps we should put a couple data sets into ggformula and use those for examples. Anyone have a new data set to suggest?