Add to the ggplot2 extensions page

[trevorld]

• https://exts.ggplot2.tidyverse.org/

• Needs stylized image

  • Perhaps ggplot2 volcano image and 3D bar chart?

• Do after CRAN release (so we can check on CRAN)

[trevorld]

Created a PR: https://github.com/ggplot2-exts/gallery/pull/163