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/tutorials/pattern-design/part3/menu/readme.mdx

@@ -0,0 +1,68 @@
+---
+title: Facilitating frontend integration
+order: 60
+---
+
+Strictly speaking, this tutorial is about learning to use FreeSewing's core
+library to do parametric design, and we made great strides in that regard.
+
+But FreeSewing is a lot more than its core library, and you might be wondering
+how your pattern options magically end up in the development environment under
+**Design options**:
+
+![Design options menu](./options.png)
+
+To make this happen, we add extra information to the options configuration.
+You can add anything you want, here is a made-up example:
+
+```mjs
+options: {
+  shipWith: {
+    dflt: 'pickUp',
+    list: ['pickUp', 'post', 'courier'],
+    menu: 'shipping',
+    extraNote: 'Pick-up Monday to Friday 10:00, to 19:00'
+  },
+}
+```
+
+It's just a silly example, but there's two important take-aways here:
+
+- You don't have to use options in your design. You can add options for things
+  that are not about the design, but that you still want to capture the user's
+  input for (like shipping preferences in this case).
+- You can add extra properties to an option. Each option type has it's required
+  properties. But you can add more and use them as you see fit.
+
+Which is exactly what we do at FreeSewing, so I'd like to mention the `menu` one:
+
+## Setting `menu` on your option
+
+If you set a `menu` property on your option, the FreeSewing frontend will use
+this to organize your various options in a menu structure.
+
+### Sub menus
+You can a nested menu structure with dot-notation. So `style.pockets` will
+create a `pockets` submenu under the `style` menu and put your option there.
+
+### Hiding options
+
+If your `menu` property holds a falsy value, the option will be hidden from the
+menu.
+
+### Hiding options conditionally
+
+If the `menu` property of your option holds a function, that function will be called with the following signature:
+
+```mjs
+function(
+  settings, // The settings provided by the user
+  mergedOptions, // The user-provided options merged with the defaults
+) {
+ // return menu value here
+}
+```
+
+This is typically used to hide options conditionally.
+
+:::note [FIXME]Include example:::