New Dex onboarding docs and support for Google Group scope

We are excited to announce that we have completely rewritten our Dex onboarding documentation. This documentation will guide you through the steps required to configure your Identity Provider (like Google, Microsoft or Github) so we can integrate it as authentication mechanism in your platform (eg. for accessing monitoring dashboards). In addition we have added support to inject extra files into the Dex configuration, which for example allows using a Google service account to limit access scopes to specific groups instead of a whole domain.