chore: Port FreeSewing.dev to docusaurus

The replaces the NextJS site powering FreeSewing.dev with a Docusaurus
setup. It's part of my efforts to simplify FreeSewing's setup so we can
focus on our core value proposition.

sites/dev/docs/reference/api/attributes/setifunset/readme.mdx

@@ -0,0 +1,27 @@
+---
+title: Attributes.setIfUnset()
+---
+
+The `Attributes.setIfUnset()` method sets the attribute identified by `key` to
+value `value` but only if it's currently unset (`undefined`).
+
+## Signature
+
+```js
+Attributes attributes.setIfUnset(string key, string value)
+```
+
+## Example
+
+```js
+const attr = new Attributes()
+  .setIfUnset('class', 'classA')
+  .setIfUnset('class', 'classB')
+
+const class = attr.get('class')
+// class now holds: "classA"
+```
+
+## Notes
+
+This will never overwrite any value and thus is a safe way to set attributes