Adjust tutorial for datapackage v2

[fkohrt]

In the tutorial, we explain in an optional note how to create a machine-readable variable documentation. The standard we are using has been published in two versions: v1 is called "frictionless data", v2 is called "data package". As of February 2025, the R package that we use (frictionless) only has support for v1, although support for v2 is planned.

Once the R package supports v2, we should update the tutorial accordingly. In particular, v2 also supports "value labels" (via categories), which v1 does not support. The tutorial currently covers value labels only for the "human-readable" data dictionary, but not for the machine-readable one.