Add docs website and revamp README

[gdalle]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

• Add docs folder with make.jl and src
• Put the public and private API reference in src/index.md
• Add Documentation.yml workflow taken from https://documenter.juliadocs.org/dev/man/hosting/#GitHub-Actions
• Revamp README
• Add badges to README

I have also configured the SSH deploy keys in the repo with DocumenterTools, as described in https://documenter.juliadocs.org/dev/man/hosting/#travis-ssh