Add a lot more general information about ggplot2

[mbcann01]

Overview

We really don't have much explanation about how to use ggplot2 in the book. There are just a couple of plots with minimal explanation. I don't think we want to cover all of the basics of ggplot2, instead we should just refer them to Hadley's book (https://ggplot2-book.org/index.html). However, learners have repeatedly asked for more information than we currently give them.

In this part:

1. Cover the very basics of the grammar of graphics and how ggplot2 works.
2. Numeric descriptions of variables

In the presenting results part:

1. Formatting ggplots and making them pretty
2. Other types of plots