Break the lab write-up into separate pages

[NicMcPhee]

This write-up is really long, which makes it intimidating, and hard to scan through for particular pieces of information. It would probably be a lot more readable if we cut it into multiple pages, with the "main" README having lots of links into those pages.

[NicMcPhee]

I had some student push-back to that idea, as they liked having it on one long thing as long as there was a good, working table of contents. Keeping it in one file also simplifies including it as a reading in Perusall. So I'm not sure I'll do this now (if ever).