Add a subsection in predictive modeling pipeline regarding missing values handling

We should add a small section to show how:

- how to handle missing data
- illustrate the usage of an imputer in a pipeline
- illustrate the usage of a pipeline within a column transformer

+ exercise

Locate the predictive modeling pipeline material in the course notebooks and read the surrounding lesson structure and exercises first. Add a subsection covering missing-data handling, an imputer in a pipeline, and a pipeline inside a column transformer, with an exercise that demonstrates these concepts.

Written by the indexing model from the issue text.