Reorganize chapters

[malcolmbarrett]

This PR:

• Renames the chapter file names to follow the pattern N-description where N is the chapter number and description is a slug for the chapter title.
• Changes all chapter titles to sentence case for consistency

The motivation here came from my recent changes to add and move chapters. It was hard to rename them and remember which was which. I kept the numbers in the file name so technically, if we move things again, we still need to change the name in two places (the file and _quarto.yml), but I find the numeric ordering very helpful so I kept that.

[malcolmbarrett]

FYI merging this @LucyMcGowan and @tgerke. Merge main on any branches you're working on sooner than later to avoid conflicts!