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/aspropsifprefixis/readme.mdx

@@ -0,0 +1,30 @@
+---
+title: Attributes.asPropsIfPrefixIs()
+---
+
+The `Attributes.asPropsIfPrefixIs()` method will return attribute values as a
+props object (a plain JavaScript object) but only for those keys who start with
+`prefix`.
+
+## Signature
+
+```js
+Object attributes.asPropsIfPrefixIs(string prefix)
+```
+
+## Example
+
+```js
+const attr = new Attributes()
+  .add('class', 'various')
+  .add('stroke', 'red')
+  .add('stroke-width', 2)
+
+const props = attr.asPropsIfPrefixIs('stroke')
+/* Props holds:
+{
+  stroke: 'red',
+  stroke-width: 2
+}
+*/
+```