Removal of vignette images to reduce package size

[JanMarvin]

Similar to the removal of most of the included example and test files, I want to remove the images from the vignettes and point people to https://github.com/JanMarvin/ox2-book for a detailed visual guide.

[olivroy]

Hi @JanMarvin, in case you were not aware,

you may want to use articles instead of vignettes

https://r-pkgs.org/website.html#non-vignette-articles

Articles only show on the website, but are not built in the pkg.

Images are valuable in docs imo.

usethis::use_article() to create one.

[JanMarvin]

Closed in #1106