add config_name and data_files to datasetcard.md specs

[lappemic]

As realised in #1222 the specs of the datasetcard are not up to date. This PR therefore updates the specs.

Let me know if this is sufficient or you would like to add more config examples.

Cc: @severo, @polinaeterna

[stevhliu]

I personally would keep it simple but not sure

Since this is just an example dataset card, we don't necessarily need to include all possible parameters. A simple example use case should suffice, with a link to more options (to allow users to explore more gradually if they'd like), as you suggested sounds great to me.