add baseline model to story notebook

[yongrenjie]

• Added rank column to df_2023.csv and df_2023_nonexploded.csv.
  • Note that rank is NaN if the country did not make it to the final.
• Added discussion of baseline model (inc. cross-validation) to story notebook.

[yongrenjie]

I'll save the merge for tomorrow, I think. 🫠

[yongrenjie]

I'm halfway cleaning code up, but I'll merge now and place that in a separate PR. Thanks for going through this @llewelld!