Tune first Foundations several segments

Add diagrams, additional slides with topic headers, diagrams, and
details labs
Add SVG logos

Git, GitHub logos for “slide” use
Add overrides for "slide" content scaling
Separate GitHub Web Flow from PR intro steps
Aggregate similar topics, add back major topic headings
Add Visualizations chapter heading
Tune title plurals
Adjust wording, additional "Detail" content
Merge pull request #226 from github/more-foundations-tuning

Update Foundations with final chapter sections and tune up