docs: Add documentation for new notifications feature

aider/website/docs/usage/notifications.md

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+---
+title: Notifications
+highlight_image: /assets/notifications.jpg
+parent: Usage
+nav_order: 760
+description: Aider can notify you when it's waiting for your input.
+---
+
+# Notifications
+
+Aider can notify you when it's waiting for your input, which is especially useful for long-running operations or when you're multitasking.
+
+## Usage
+
+Enable notifications with the `--notifications` flag:
+
+```bash
+aider --notifications
+```
+
+When enabled, aider will notify you when the LLM has finished generating a response and is waiting for your input.
+
+## OS-Specific Notifications
+
+Aider automatically detects your operating system and uses an appropriate notification method:
+
+- **macOS**: Uses `terminal-notifier` if available, falling back to AppleScript notifications
+- **Linux**: Uses `notify-send` or `zenity` if available
+- **Windows**: Uses PowerShell to display a message box
+
+## Custom Notification Commands
+
+You can specify a custom notification command with `--notifications-command`:
+
+```bash
+aider --notifications-command "your-custom-command"
+```
+
+For example, on macOS you might use:
+
+```bash
+aider --notifications-command "say 'Aider is ready'"
+```
+
+## Configuration
+
+You can add these settings to your configuration file:
+
+```yaml
+# Enable notifications
+notifications: true
+
+# Optional custom notification command
+notifications_command: "your-custom-command"
+```
+
+Or in your `.env` file:
+
+```
+AIDER_NOTIFICATIONS=true
+AIDER_NOTIFICATIONS_COMMAND=your-custom-command
+```