[Data Science] Rework Linear Regression (OLS) Chapter

[JakobKlotz]

Overhauled the Linear Regression Chapter:

• Use scikit-learn throughout
• Demonstrate the model using the California Housing data
• Introduced a "typical sklearn" workflow (train test split, fit, predict, evaluate)

Some chores:

• Fixed an unreadable plot in the random forest chapter, when using the light mode
• Put some plots in a dedicated subfolder in the assets/ dir