deploying via GitHub Actions

[nhejazi]

There's been a surge of migration from deploying bookdown via Travis to deploying via GitHub Actions, which has also resulted in a collapse in support for using Travis to this end (note: this change is largely due to management decisions at Travis). To limit future overhead, we should consider moving to GitHub Actions, which is supported by a combination of the tic package, the usethis package, and ROpenSci's documentation.

[nhejazi]

For example, this GH-actions setup is supposed to basically work out-of-the-box, usethis::use_github_action(url = "https://raw.githubusercontent.com/ropenscilabs/actions_sandbox/master/.github/workflows/deploy_bookdown.yml")

[nhejazi]

For reference, see the successful working example at https://github.com/tlverse/enar2021-workshop/blob/master/.github/workflows/bookdown.yml