Change color of code designation

This commit changes the left bar that runs in the margin of any
code from light gray to black.
Reorder some of the linear regression material

This commit makes an 'interpretation of linear regression' 'part'
of the book that includes the material on modeling and on causality.
It sort of serves the purpose of 'advanced topics' on regression.
Tweak URL formatting

- Adds a quote-style '<' and '>' to urls.