[WIP] Update three-part class slide decks

:warning: Work in progress :warning: 

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
- [ ] Establish new chapters (Jekyll Tags) identifying "stories" not just Git commands
- [ ] Utilize additional (new) diagrams for clarifying concepts
- [ ] Rebuild all of *Foundations*
- [ ] Rebuild all of *Intermediate*
- [ ] Rebuild all of *Advanced*