Adds motivating example to RobustRegression notebook

[zaxtax]

Motivation

This uses scikit-learn to show what happens when a regular regression is used and motivates the need for robustness in some examples with a plot.

Changes proposed

Adds some code to fit a regression using scikit-learn and an additional plot

Types of changes

• [x] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] The title of my pull request is a short description of the requested changes.