Document how to use with GitHub Actions

[fulldecent]

Fixes #2242

[jaapio]

Thanks for creating this part of the docs. It's really nice that you are adding fixes for the docs. It would be nice if you could check the github action, I think this helps to make the workflow easier.

[jaapio]

phpDocumentor provides a github action from this repository: https://github.com/phpDocumentor/phpDocumentor/blob/master/action.yml

      - name: "Build"
        uses: "phpDocumentor/phpDocumentor@master"
        with:
          target: "build/docs"

I think this is a better way of running since we can pack every dependency we need in a docker image. So people do not have to care about installing php correctly.