New shell beginner lessons

Work in progress: this PR brings in a half day lesson on the Unix shell aimed at people who've never seen it before. What it *doesn't* bring in is:

*   diagrams
*   styling
*   a single `make`-style command to build the pretty versions of things
*   metadata (e.g., instructor's notes and summaries for students)

Please comment on the content while we work on those things.