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/add/readme.mdx

@@ -0,0 +1,25 @@
+---
+title: Attributes.add()
+---
+
+The `Attributes.add()` method adds `value` to the attribute identified by
+`key`.
+
+## Signature
+
+```js
+Attributes attributes.add(string key, string value)
+```
+
+## Example
+
+```js
+const attr = new Attributes()
+  .add('class', 'classA')
+  .add('class', 'classB')
+```
+
+## Notes
+
+Adding multiple values to the same key will result in them being joined together
+(with a space) when rendering.