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/path/angleat/readme.mdx

@@ -0,0 +1,54 @@
+---
+title: Path.angleAt()
+---
+
+The `Path.angleAt()` method returns the (tangent) angle of a path at a specific point.
+
+If the given point is a sharp corner, this method prefers returning the angle directly before the corner.
+
+If the given point does not lie (approximately) on the path, this method returns `false`.
+
+## Signature
+
+```js
+number|false path.angleAt(Point point)
+```
+
+## Example
+
+<Example caption="Example of the Path.angleAt() method">
+```js
+({ Point, points, Path, paths, snippets, Snippet, part }) => {
+
+points.A = new Point(45, 60)
+points.B = new Point(10, 30)
+points.BCp2 = new Point(40, 20)
+points.C = new Point(90, 30)
+points.CCp1 = new Point(50, -30)
+points.D = new Point(50, 80)
+points.DCp1 = new Point(70, 30)
+
+paths.demo = new Path()
+.move(points.D)
+.curve(points.DCp1, points.DCp1, points.C)
+.curve(points.CCp1, points.BCp2, points.B)
+.line(points.A)
+
+points.testPoint = paths.demo.shiftFractionAlong(0.55)
+snippets.point = new Snippet("notch", points.testPoint)
+
+let angle = paths.demo.angleAt(points.testPoint)
+//draw a tangent path
+paths.tangent = new Path()
+  .move(points.testPoint.shift(angle, -30))
+  .line(points.testPoint.shift(angle, 30))
+  .attr("class", "lining dashed")
+
+return part
+}
+```
+</Example>
+                               
+## Notes
+
+Keep in mind that calculations with Bézier curves are often approximations.