Content preview for Cardgroup element

# Feature Request

Currently the backend shows the cardgroups as they are regular containers.
But the cardgroup element does not show any cardgroup items in the backend page view.

## Is your feature request related to a problem? Please describe

No, it is simply nice to have to optimize editorial workflows.
Right now, when I use several cardgroup elements on one page I have to guess in which cardgroup element I would like to change a cardgroup item.

## Describe the solution you'd like

When in page view mode, I see all cardgroup items in the cardgroup element.

## Describe alternatives you've considered

it stays the way it is.

## Additional context