Move iqss demo to vignettes

[izahn]

Now that the IQSS theme no longer has a hard dependency on Libertinus fonts it can be moved to the vignettes directory. Maybe a good idea to wait to move it until the demo is cleaned up: IQSS/iqss-beamer-theme#19

[eddelbuettel]

In the package we actually do have two documents: the skeleton.Rmd everybody gets via the RStudio dialogue, or equivalent code. That probably wants to be short-ish. And then we have the vignette where we can show off more. I think for all three themese we can round things off a little (as needed, not too urgent).