feat(docs): Add background guide Flutter

src/code-snippets/flutter/examples/AdjustConfig/sendInBackground.mdx

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+```dart {3}
+AdjustConfig adjustConfig = new AdjustConfig('{YourAppToken}', AdjustEnvironment.Sandbox);
+//...
+adjustConfig.sendInBackground = true;
+//...
+Adjust.start(adjustConfig);
+```

src/code-snippets/flutter/signatures/AdjustConfig/sendInBackground.mdx

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+```dart
+bool? sendInBackground;
+```

src/content/docs/en/sdk/flutter/configuration/background.mdx

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+---
+title: Record activity in the background
+description: Configure the SDK to send requests in the background.
+slug: en/sdk/flutter/configuration/background
+---
+
+By default, the Adjust SDK pauses the sending of requests when your app is running in the background. You can configure the SDK to send requests in the background by enabling the background recording feature.
+
+import SetSendInBackgroundSig from "@flutter-signatures/AdjustConfig/sendInBackground.mdx"
+
+Your config object contains a boolean <Tooltip>`sendInBackground` property || <SetSendInBackgroundSig /></Tooltip> that controls this behavior. When set to `true`, the SDK sends data to Adjust while the app is running in the background.
+
+<Callout type="important">
+
+You must set the `sendInBackground` property *before* you initialize the Adjust SDK.
+
+</Callout>
+
+import SetSendInBackground from "@flutter-examples/AdjustConfig/sendInBackground.mdx"
+
+<SetSendInBackground />