Discussion of regression should be moved to later in the workshop.

The following paragraph (at https://carpentries-incubator.github.io/machine-learning-trees-python/02-decision-tree/index.html) is forward referencing. To understand the paragraph, we need to understand concepts that haven't yet been taught.

> While we will only be looking at classification here, regression isn’t too different. After grouping the data (which is essentially what a decision tree does), classification involves assigning all members of the group to the majority class of that group during training. Regression is the same, except you would assign the average value, not the majority.

We should move the discussion of regression to later in the content.