CLI slides

Did a presentation to cover what's in the command line page of the upstream docs, plus a few more commands, and concepts and history. Probably still too much of an info-dump. Moved the old command line page to a sort of "extra credit" section since it had some unrelated stuff, so we can take whatever needs to be cut for time and move it there. Feedback appreciated! Anyone else is of course welcome to make major edits or run with presenting it.
FYI, there's a discussion "going on" about separating the command line material as its own curriculum: https://github.com/railsbridge/docs/issues/335