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/plugins/ringsector/readme.mdx

@@ -0,0 +1,30 @@
+---
+title: plugin-ringsector
+---
+
+Published as [@freesewing/plugin-ringsector][1], this plugin facilitates
+drafting a ring sector (like a part of a donut).
+It is particularly usefor for drafting curved waistband, circle skirts, and so on.
+
+## Provides
+
+- [The ringsector macro](/reference/macros/ringsector)
+- [The rmringsector macro](/reference/macros/rmringsector)
+
+## Installation
+
+```sh
+npm install @freesewing/plugin-ringsector
+```
+
+## Usage
+
+You should [add it as a part plugin](/reference/api/part/config/plugins).
+Refer to the documentation of [the provided macros](#provides) for details on how to use them.
+
+To import the plugin for use:
+```js
+import { plugin as ringsectorPlugin } from '@freesewing/plugin-ringsector'
+```
+
+[1]: https://www.npmjs.com/package/@freesewing/plugin-ringsector