Update three-part class slide decks

This considerably overhauls the *Foundations*, *Intermediate*, and *Advanced* class slides. Changes include:

* Improved narratives and topic order
* Common and most-useful command examples
* Better workflow scenarios

### Tasks
- [x] Establish new chapters (Jekyll Tags) identifying "stories" not just Git commands
- [x] Utilize existing (but possibly unused `slides/assets/diagrams`) visuals for clarifying concepts
- [x] Rebuild all of *Foundations* (as of c7f6f12)
- [x] Rebuild all of *Intermediate* (as of 8b190e9)
- [x] Rebuild all of *Advanced* (as of f789177)