introductory exercises answers hidden built

[adokter]

The introductory exercises vignette (rad_aero_19.Rmd) contains a flag by which you can toggle whether to generate a vignette showing the anwers or not, i.e. by settingSHOW_ANSWERS to TRUE or FALSE.

We want to show the vignette with SHOW_ANSWERS=FALSE in the pkgdown site, and included in the articles section of the site.

We also want to generate the vignette with SHOW_ANSWERS=TRUE, but in that case not included in the articles section, but only as a (relatively hidden) page accessible by URL.

Figure out how to do this automatically and efficiently.