Adds figures to documentation and script to automatically update them as needed

[karissawhiting]

Adds figures to docs and script to generate them. Does not update README figures yet. Figure sizes may need to be adjusted. Re: Issue #27

[ddsjoberg]

OMG this PR is so so amazing! I can't wait to review :)

[ddsjoberg]

The tables look amazing! The code you wrote to create them is so short and easy to follow!

The checks all run without error/warning, and the help files all contain the amazing tables. A few initial thoughts (all of these can be addressed in a separate PR):

1. The tables look especially nice on the {pkgdown} site :)
2. I think we should shorten the tbl_summary table examples (and perhaps others). Also, two examples is should be sufficient (rather than the three we have). With fewer examples and shorter tables, the space between example code and the rest of the help file will be shorter. This requires no change before merging the pull request. We can make a separate issue to make these kinds of updates.
3. Rather than call the section "Figures", perhaps we should call it "Example Output"? That may help people understand more easily that these are the output tables.
4. Is there a way to make a separation between two example tables? Maybe an html break, <br>?
5. This last one is a me issue I've had with {webshot}. When I try to run the code, I get an error. Do you know how to resolve it?