Fixes #206 restore vignette image files

[pchelle]

Fix suggested from r-lib/pkgdown#1231

[IndrajeetPatil]

Nice! Thanks for taking care of this.

[pchelle]

Nice! Thanks for taking care of this.

I am not sure this will work though. In case I found this as backup plan (but I still lack experience with github actions):
ArgoCanada/argoFloats#172

[Yuri05]

I hope the page will be displayed black on white background and not white on black background? 😉

[pchelle]

I hope the page will be displayed black on white background and not white on black background? 😉

Should be good for {tlf}, there is still some black theming in the reporting engine documentation though.
Comparing the _pkgdown.yml files with the other packages, there is still some theme: arrow-dark among the template features