Use pkgdown to make a website for the package

[ben18785]

This seems to suggest that we can produce static HTML versions of our vignettes.

[hfldai]

@ben18785 This suggests we can use pkgdown::build_site() and easy manual set up in [Settings] to create the HTML github pages on the generated /doc folder. It requires manual update for docs/ whenever we have a local change, but much easier than the heavy setups in using usethis::use_pkgdown_github_pages() for automatically documentation. Only thing is that the access to [Settings] seems to be restricted to the owners of the repo.

[ben18785]

Closed by #62