Host gradethis demo online

[garrettgman]

gradethis_demo() launches a learnr tutorial locally that demos aspects fo gradethis for users. Some users are using the demo as a reference, which becomes awkward because they want to view it at the same time that they are hosting locally a tutorial that they are in the progress of writing. We can make life easier for these users by deploying a copy of gradethis demo to shinyapps.io. Bonus points if we can also:

• Insert the demo into the repository readme as an iframe
• Write a script that automatically redeploys the demo when it changes

[schloerke]

For deployment, we should mirror what learnr does: https://github.com/rstudio/learnr/blob/dfe7401e4e6916461819ba31061dd3801f088e92/.github/workflows/R-CMD-check.yaml#L102-L158

For tutorials, we should include in the pkgdown config (when it's added) like sortable: https://github.com/rstudio/sortable/blob/cdde09d915ee0bc73353b079c7afbed8cb70ab4a/_pkgdown.yml#L38-L41