Add pkgdown site

[hturner]

Possibly overkill for this package, but maybe not when we have added more vignettes/functionality!

The content is automatically generated from the README, vignette, helpfiles, etc, so it should not be a big task to create one based on the defaults at least.

We can use GitHub actions to automatically rebuild it for us when something changes:
https://github.com/r-lib/actions/tree/master/examples#build-pkgdown-site

[hturner]

Helper functions to use GitHub actions: https://usethis.r-lib.org/reference/github_actions.html