Retire the Loop in the documentation

The first thing that a new user sees when stumbling across the QCoDeS documentation is how to set up a measurement with the old, hopefully soon-to-be-retired Loop. That should not be so. I propose that we shuffle around the documentation a bit and in particular make the `Tutorial.ipynb` and the `docs/index.rst` use the new dataset and context manager.

Read docs/index.rst and Tutorial.ipynb to understand how the documentation currently introduces measurements and the old Loop. Update both entry points to use the new dataset and context manager, then verify that a new user no longer encounters the old Loop as the initial setup path.

Written by the indexing model from the issue text.