chore(markdown): Restructuring dev docs

markdown/dev/reference/api/settings/units/en.md

@@ -0,0 +1,15 @@
+--- 
+title: units
+---
+
+Either `metric` (the default) or `imperial`.
+
+Note that this is only used to format the output. Freesewing expects mm.
+
+```js
+import brian from "@freesewing/brian";
+
+let pattern = new brian({
+  units: "imperial"
+})
+```