Set up development mode pkgdown docs

[jayqi]

pkgdown supports having both a release copy of documentation (in docs/) and a development version of documentation (in docs/dev/).

We can even have it infer automatically which to build based on the version number. This would involve setting it in https://github.com/uptake/pkgnet/blob/master/_pkgdown.yml

Documented here: https://pkgdown.r-lib.org/reference/build_site.html#development-mode