Consider moving the pkgdown process and docs to separate workflows, and update deployment

[johnkerl]

Follow-up from #797 (comment)

[jdblischak]

Some notes:

• Current manual process to update documentation is in wiki page Building the Package Documentation
• The example GitHub Actions workflow for pkgdown deployment from r-lib/actions demonstrates the step for pushing to the gh-pages branch. We should be able to simply copy-paste the last two steps (plus a step to install pkgdown). The earlier setup steps using r-ci.sh can be copied from ci.yaml
• For the vignettes, the source files are the .md files, which then generate the .Rmd files via vignettes/Makefile. If we want to keep this system, the new docs workflow should run cd vignettes && make. If we prefer to use Rmd as the source, then we should delete the .md files and Makefile to reduce the amount of duplication in the repo
• To have both a release and development version of the docs, we can specify mode: auto for the Development mode

development:
  mode: auto