docs: improve challenge 06 setup

[heddendorp]

No description provided.

[georgiee]

So you head problems with CDK? It's in the package.json from the very beginning. I don't know what happened. I would suggest you mention the error — so anyone with the same problem can run npm install while it should work for most people. Can you change the PR? Thanks a lot for spotting this.

See that branch from the first challenge - it's there ?

https://github.com/georgiee/skipbo-angular/blob/7f0d8ade67009dc3983b846dfc3cf675c1132053/package.json#L16

[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.