This action is currently only enabled for the ropensci universe. It builds the pkgdown site from a git url and saves the entire site it in a zip file. It part of the package build workflow that runs for each package update in ropensci. The action automatically takes care of dependencies.
You can test it locally like this:
docker run -it ghcr.io/r-universe-org/build-docs "https://github.com/jeroen/jsonlite"