[FIX] Improve `Introduction to Hierarchical Forecasting`

[AzulGarza]

The Base predictions and Reconciliation sections don't have a complete explanation of the hierarchical reconciliation methodology. Ideas to improve those sections:

Base predictions:

• Define what we mean by "base predictions".
• Use a different model. By construction, the Naive model is coherent, so this model does not well motivate the use of hierarchical reconciliation. In particular, RandomWalkWithDrift could work.

Reconciliation

• Show that the base forecasts are not coherent, using plot_hierarchical_predictions_gap (fix #85 first).
• Explain the methods used for reconciliation. In this case, the explanation for BottomUp is missing.

[El-Don-Quijote]

Thank you for raising this one, I just noticed the same thing. Some difference between the Naive and the Naive/Bottum Up forecasts would be more illustrative.