Developer website deployment scrubdown
Thank you @jelledelaender 🙇 

@jedrichards to finish the job next week:

- [x] remove `gh-pages` branch and whatever else is left over from GitHub pages
- [x] update `README`
- [x] remove redundant deploy scripts
- [x] remove redundant `package.json` script entries
- [x] document release process internally

_Originally posted by @christianklotz in https://github.com/BohemianCoding/SketchAPI/issues/607#issuecomment-524279780_