Update the headers to each .ino to reference textbook, as relevant

[jonfroehlich]

Update the headers to each .ino to include:

• Specific assumptions about hardware
• Wiring
• Link back to the textbook chapters, as relevant (https://makeabilitylab.github.io/physcomp/)

[jonfroehlich]

Scope note: this overlaps #13 (the fundamental code + comment review pass). When picking this up, decide whether to fold it into #13 or keep it as a focused headers-only task. Tracked in the roadmap, #17.