Add OverlayDevice reference

Author: AtibQur

sites/cheerpx/src/content/docs/12-reference/CheerpX.OverlayDevice/create.md

@@ -0,0 +1,18 @@
+---
+title: create
+description: Virtual filesystems and how to use them
+---
+
+```ts
+namespace CheerpX {
+  class OverlayDevice {
+    static async create(baseDevice: Device, overlayDevice: Device): Promise<OverlayDevice>;
+  }
+}
+```
+
+## Parameters
+
+- **baseDevice (`Device`)** - The underlying device (e.g., `HttpBytesDevice`, `IDBDevice`) that serves as the base layer for the filesystem.
+
+- **overlayDevice (`Device`)** - The writable layer that will overlay the base device, enabling persistent changes.