change ggplot examples

[ErinBecker]

Episode 6 is currently titled "Creating Publication-Quality Graphics". The original episode that this was copied from went into a lot more detail and actually did end with the learners creating publication quality graphics. However, we've cut this episode down a lot and the graphics the learners make are not anywhere near publication quality. This isn't a problem - we have an episode in the raster vector lesson (https://datacarpentry.org/r-raster-vector-geospatial/13-plot-time-series-rasters-in-r/index.html) that goes into a lot of detail and makes a really nice publication-quality graphic. However, it does mean that we should change some things in this episode to update it to the current content:

• [ ] rename the episode to something like "Introduction to Visualization"
• [ ] add a note at the end of the episode stating that the next lesson in the workshop (raster vector lesson) goes into a lot of detail about plotting and any questions not answered here will hopefully be answered there. Include a bulleted list of what they will learn about ggplot in the raster vector lesson?
• [ ] add a note at the beginning of the episode stating that this episode covers just the bare essentials in ggplot and that a lot more is coming in the raster vector lesson.
• [ ] condense the short paragraphs at the start of the episode to one paragraph
• [ ] update the plotting examples so that they create graphics that are meaningful without further modification. These examples work well in the original lesson, because they are further refined with faceting and other tricks, however they aren't interpretable in their current form.

[ErinBecker]

The raster vector lesson uses the following geoms (other than geom_raster and geom_sf):

• geom_histogram
• geom_bar
• geom_point
• geom_line (last episode only)

These geoms appear in that lesson in the order listed above. Since geom_line and geom_point show up so late in that lesson, it seems to make more sense to have the examples in this introductory episode use geom_histrogram and/or geom_bar. I will work on a PR with some suggested examples.

[lachlandeer]

Seems like #52 fixes this. Since it's merged in I'm gonna close it