ci: add docs building and hosting

[dshemetov]

Copied example from https://github.com/r-lib/actions/tree/v2/examples#build-pkgdown-site

[brookslogan]

Closes #44. I'll try to review later today.

[brookslogan]

For uniformity/convenience, we should try to use usethis for all our CI, etc., rather than manually copy-pasting files. Here, we can use

usethis::use_github_action("pkgdown")

This has a slightly different name for the workflow file (committed above). (It also adds a .github/.gitignore ignoring .html files (maybe from r-lib/usethis#855) --- I'm not sure I've run into html files being placed there, but maybe it could be from something like nektos/act.)