docs: Document the new Install group support

[alexlarsson]

@XenGi look ok?

[XenGi]

@alexlarsson Looks good. I would just add a note that if you want to add a dependency to another quadlet generated container that you have to use the .service extension and not the .container extension to avoid confusion.

[XenGi]

I guess after that it's time for a new release. :D

[b-m-f]

Thanks alot @alexlarsson