JC edits

[jcumby]

1. Updated the style guide to include an "Overview" section at the top of the notebook
2. Updated content in Unit 01 and Unit 10 to:
   • Use correct heading levels
   • Remove erroneous content
   • Update figures to license-free versions
3. Updated Mentimeter image in paper

[ppxasjsm]

Looks great! Thanks for going through this :)