Add screenshots or a short walkthrough to the README

Right now someone evaluating this project has to either read through the docs or stand up their own instance (`SETUP.md`) to see what it actually looks like. A few screenshots of the public home page, the intake form, and the token portal (or a short screen recording/GIF) would let a prospective adopter judge fit in under a minute. ReCARES's own live deployment is linked from the README's "See it in action" section and can be used as the source for screenshots, since it is a real, populated instance (please do not submit test data there; if you need a populated demo to screenshot, stand up your own instance per `SETUP.md` and seed it with a few test submissions instead). This does not require writing code, so it is open to anyone comfortable putting together a few images or a short recording, not just people who want to touch the Apps Script source.