Update GitHub Actions themselves, not just embedded dependencies

[brookslogan]

usethis::use_github_action("pkgdown")
usethis::use_github_action_check_release() # and follow instructions regarding badge

[brookslogan]

(Migrating note to GitHub:) It looks like the updated pkgdown Action will try building the site for PRs and give us a heads up about failures before they are merged into main. The tradeoff is that this likely involves building the vignettes another time (I think current checks build the vignettes twice, so this would be a third time), meaning potentially more time spent waiting on Actions to complete.