Update README files to include new features and improvements

Author: lijy91

docs/index.md

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+# Introduction
+
+The `window_manager` plugin provides comprehensive window management capabilities for Flutter desktop applications, enabling full control over window size, position, appearance, close behavior, and listening to events.
+
+## Core Features
+
+### Window Control
+
+- Set size/min/max limits
+- Position windows across displays
+- Manage states: maximize/minimize/fullscreen
+- Custom close handlers
+
+### Visual Customization
+
+- Hide title bars
+- Create borderless windows
+- Adjust opacity/background color
+- Control window shadows
+
+### Event Listeners
+
+- Lifecycle: open/close events
+- State: maximize/minimize/fullscreen
+- Position: move/resize
+- Focus: focus/blur
+
+## Platform Support
+
+| Platform | Support            |
+| -------- | :----------------- |
+| Linux    | ✔️ Fully supported |
+| macOS    | ✔️ Fully supported |
+| Windows  | ✔️ Fully supported |