CSCS: Fix "Python virtual environments with uenv"

[AnnikaLau]

The section Python virtual environments with uenv in the CSCS docs needs improvement.

The correct way is now in the docs under Create Python image on Säntis. CSCS is theoretically informed, but they don't seem to be doing anything about it.

[mjaehn]

Theoretically, we could open a PR for the CSCS docs. What do you think?

[AnnikaLau]

@leclairm did that already. I don't know what happened with it and if it fixed everything.