Revising slide content, notes, titles

Changes are _not_ PR merge-ready as of first commit, but wanted to open up the review and discussion around the slide deck content.

Goals of this PR and the associated changes:

* Improve the language and consistency across slides
* Tidy up redundant content
* Amp-up the presenter notes and align with class notes/outline

Slide deck optional goals:

* Fix basic order of slides to more gradually build knowledge basic for students
* Resolve topics that are ahead of each other (e.g. `git checkout branch` before `git branch`, etc.)