TabPFN Example and Alignment with Paper

[mmschlk]

This PR extends the documentation by adding an example notebook on how to use the new TabPFN model and conductes some maintanance:

Changelog:

• adds the TabPFN Notebook
• renames game_fun parameter in shapiq.ExactComputer to game and closes Refactor game_fun in ExactComputer? #297
• removes warning when class_index is not provided in explainers and closes We can't have a warning by default for regression models #298
• runs and updates the example notebooks

[FFmgll]

LGTM :)