support branch deploys for static website hosts

[maxheld83]

would be really nice to deploy arbitrary branch foo to foo.project.com, which is what netlify currently offers but only if you use their build infrastructure (which obviously doesn't work for R, but only Hugo and friends).

• netlify cli support is tracked here: Set Deploy Context / Branch from CLI netlify/cli#275
• github pages will not support this (unless foo/ subdirectory, though that's weird/dangerous/awkward).
• firebase ?!? https://stackoverflow.com/questions/31195694/attempted-to-deploy-to-firebase-on-github-branch
• surge? https://surge.sh/
• aws?

pkgdown once discussed/wanted this in r-lib/pkgdown#428.