Refactor setup documentation

docs/routes/usage/illustrations.md

@@ -0,0 +1,21 @@
+---
+title: Using illustrations
+---
+
+Find an illustration from [unDraw](https://undraw.co/illustrations) and download it as an SVG.
+
+Put the illustration in the `src/assets/images/illustrations` folder. Rename it to `undraw_` + the illustration slug.
+
+Replace colors in the SVG with CSS variables such as `var(--color-brand)` and `var(--color-raised)`. For colors that are the same as the font color, use `currentColor`.
+
+Add the `.illustration` class to the SVG
+
+Import the SVG in the `<script>` of your svelte file, and treat the illustration as a Svelte component:
+
+```svelte
+<script>
+	import NoData from '$assets/images/illustrations/undraw_no_data.svg'
+</script>
+
+<NoData />
+```