Add Additional Practice Problems

[hschafer]

We have a bunch of other practice problems that didn't get included in the main lessons. We should copy these over at some point.

A couple of notes on implementation details:

• This might be a case that trying to programmatically create the problems like we did with the initial book conents might be too hard. Would need to modify the lesson sync script to include practice problems and would need to change the generate_pages.py to somehow ignore pages we've already generated (not impossible, just extra work).
• Maybe would make sense to embed each practice problem in the lesson they belong to. Maybe make a folder in each lesson that we link to at the end of the lesson's Table of Contents (clearly indicating they are extra practice). That keeps things together with its content but can keep things less cluttered. So an example folder structure coul be

module/
  lesson/
    index.md   # Table of contents
    slide-1.md
    slide-2.md
    practice-problem.md
    practice-probblem.zip
    optional-practice/
      index.md  # Table of contents
      prob-1.md
      ...