Move widgets out of builtins/ and into its own section under the language reference

docs/reference/src/language/widgets/progressindicator.md

@@ -0,0 +1,23 @@
+## `ProgressIndicator`
+
+The `ProgressIndicator` informs the user about the status of an on-going operation, such as loading data from the network.
+
+### Properties
+
+-   **`indeterminate`**: (_in_ _bool_): Set to true if the progress of the operation cannot be determined by value (default value: `false`).
+-   **`progress`** (_in_ _float_): Percentage of completion, as value between 0 and 1. Values less than 0 or greater than 1 are capped.
+
+### Example
+
+```slint
+import { ProgressIndicator } from "std-widgets.slint";
+export component Example inherits Window {
+    width: 200px;
+    height: 25px;
+    ProgressIndicator {
+        width: parent.width;
+        height: parent.height;
+        progress: 50%;
+    }
+}
+```