Clea up vignette directory

[ChristopherMancuso]

there is a lot of images in the vignette directory, maybe we can talk about how best to clean that up a bit?

[Andrew0Hill]

I can work on this.

When I wrote the BIOS cluster vignette I tried to make a separate directory for all the screenshots, but there was a bug in pkgdown (r-lib/pkgdown#2807) that was causing image paths with slash characters to get corrupted when making the HTML pages (causing the images to not show up at all in the HTML version of the article if they were in a different directory than the main article), which is why everything is a mess.

The pkgdown bug should be fixed, but I don't know if the fix has made its way into a pkgdown release yet.

I'll experiment with this sometime soon and see if the bug still happens.