docs: add mirai opacity widget markdown

website/docs/widgets/opacity.md

@@ -0,0 +1,41 @@
+# Opacity
+
+Mirai opacity allows you to build the Flutter Opacity widget using JSON.
+To know more about the Slider widget in Flutter, refer to the [official documentation](https://api.flutter.dev/flutter/widgets/Opacity-class.html).
+
+## Properties
+
+| Property | Type                   | Description                                                                                          |
+| -------- | ---------------------- | ---------------------------------------------------------------------------------------------------- |
+| opacity  | `double`               | The opacity value between 0.0 and 1.0 which controls the visibility of the child, with 0.0 being fully transparent (invisible) and 1.0 being fully opaque (visible).                                                                                          |
+| child    | `Map<String, dynamic>` | The child widget of the opacity.                                                                     |
+
+## Example JSON
+
+```json
+{
+  "type": "scaffold",
+  "appBar": {
+    "type": "appBar",
+    "title": {
+      "type": "text",
+      "data": "Opacity"
+    }
+  },
+  "body": {
+    "type": "center",
+    "child": {
+      "type": "opacity",
+      "opacity": 0.5,
+      "child": {
+        "type": "text",
+        "data": "Opacity Widget",
+        "style": {
+          "fontSize": 23,
+          "fontWeight": "w600"
+        }
+      }
+    }
+  }
+}
+```