chore: Update documentation (#8)

This PR includes several minor changes to configuration:
- `.Rbuildignore` now ignores two additional files previously being
noted by R CMD check
- `_pkgdown.yml` now includes basic custom sections for the reference
section
- `_pkgdown.yml` and `DESCRIPTION` now contain the GitHub Pages URL

Further, this PR updates README.md code examples to be up to date with
the latest released version. Selected variables changed slightly within
the older sections, but did not affect the overall conclusions.

The updated README also highlights an issue with how some functions are
called (such as requiring an empty `data.frame` even when no forest is
being created), which I intend to address in the future.
For now, the updated README will at least provide guidance as to how the
function can be called successfully.