Update Quickstart ... for scs2.

[garloff]

The documentation needs adjustment for scs2 (and some of the adjustments would have been nice even before already), see #238 (comment)

• CSPO no longer needed (as you say)
• envsubst and csp-helper helm chart no longer needed
• We don't pull from github (but from our registry), so not PAT needed (that was used to prevent throttling)
• The examples (e.g. cluster-class variables) should use scs2 these days