Document using -symbolic icons

### Problem

I've seen a couple times developers including gresourced icons in dark and light variants instead of using the -symbolic namespace. We don't current document this so it's not obvious that this is a built-in feature

### Proposal

Document:
- [ ] using `-symbolic` namespace for automatic dark/light icons
- [ ] coloring `-symbolic` icons using `.accent` in Gtk.CSS
- [ ] coloring `-symbolic` icons using semantic colors like `error` or `success`
- [ ] coloring `.accent` icons using named colors like `red` or `green`

### Prior Art (Optional)

_No response_