Have you read the documentation I linked? You don’t need to build on GitHub if you use Automated builds. You can use hooks on Docker Hub. You just need to create them in your git repo.
Yes, because it is your choice whether you want to build on GitHub and push to Docker Hub or do everything on Docker Hub.
There is only one thing I could not solve on Docker Hub and this is scheduled nightly builds for specific branches, so I use CircleCI for that until it is implemented on Docker Hub.