docs: improve challenge 06 setup

[heddendorp]

[heddendorp]

I've gone through the branches again and found a second thing. rxjs-spy is not found in the start branch and even though the cdk is already present cdkDropListDisabled was not available until #13722 and version 7.1 => cause of the crash when not running npm install

Additionally i would consider only having the src folder in a sub module to make dependency management easier or going even further, only have a WorkshopModule and have the frame application be static through the workshop challenges.