Documentation: improve API documentation

[PGijsbers]

When on API documentation, you should not directly see Python API, but see menu / cards with all the APIs

In the minimal API example, on https://www.openml.org/apis, it's not very clear how the API key works. If we include a .publish() in this script, we should add the steps to connect to the test server.

documentation on how to work on the documentation: https://docs.openml.org/OpenML-Docs/