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/sa/readme.mdx

@@ -0,0 +1,77 @@
+---
+title: Adding seam allowance
+order: 10
+---
+
+When adding seam allowance to a pattern, there are 3 things that come into play:
+
+- Does the user want seam allowance?
+- How much seam allowance does the user want?
+- How can we add seam allowance?
+
+Let's whip up a quick example for a moment to clarify how we will handle this:
+
+<Example settings="sa: 10"caption="Some straight lines and some curves">
+```design/src/bib.mjs
+function draftBib({
+  Path,
+  Point,
+  paths,
+  points,
+  // highlight-start
+  sa,
+  // highlight-end
+  part,
+}) {
+
+  points.topLeft = new Point(0,0)
+  points.bottomRight = new Point(100,40)
+  points.topRight = new Point(points.bottomRight.x, points.topLeft.y)
+  points.bottomLeft = new Point(points.topLeft.x, points.bottomRight.y)
+  points.cp1 = new Point(50, 20)
+  points.cp2 = new Point(70, 60)
+
+  paths.shape = new Path()
+    .move(points.topLeft)
+    .line(points.bottomLeft)
+    .curve(points.cp1, points.cp2, points.bottomRight)
+    .line(points.topRight)
+    .line(points.topLeft)
+    .close()
+    .addClass('fabric')
+
+  // highlight-start
+  if (sa) paths.sa = paths.shape.offset(sa).addClass('fabric sa')
+  // highlight-end
+
+  return part
+}
+```
+</Example>
+
+As you can see from the source, we can destructure an `sa` variable (short for
+seam allowance) that will hold either:
+
+- `false` if the user does not want seam allowance
+- A value in `mm` indicating how much seam allowance the user wants
+
+To add seam allowance to our path, we just `offset` it by `sa` and add
+some classes to it to style it. But, crucially, only if the user wants
+seam allowance:
+
+```mjs
+if (sa) paths.sa = paths.shape
+  .offset(sa)
+  .addClass('fabric sa')
+```
+
+To refer back to our three question: Whether the user wants seam allowance, and
+if so how much seam allowance is answered by the `sa` value passed to our draft
+method.
+
+And we add it with the `path.offset()` method which will offset the path.
+
+Our bib pattern does not require seam allowance as it will be finished with
+bias tape. But we can use this same technique to indicate that.
+
+Before we do so though, we should talk about `complete`.