Let's say you want a certain key in your layout to send a colon; to figure out what keycode to use to make it do that, you're going to need `quantum/keymap_common.h`.
That file contains a big list of all of the special, fancy keys (like, being able to shend % on its own and whatnot).
If you want to send a plain vanilla key, you can look up its code under `doc/keycode.txt`. That's where all the boring keys hang out.
