enhancement: mention SentryWidget when setting up Flutter (#12390)

* enhance, mention sentrywidget when setting up sentry

* improve replay docs

Author: vaind

docs/platforms/flutter/index.mdx

@@ -62,7 +62,11 @@ Future<void> main() async {
       // Note: Profiling alpha is available for iOS and macOS since SDK version 7.12.0
       options.profilesSampleRate = 1.0;
     },
-    appRunner: () => runApp(MyApp()),
+    appRunner: () => runApp(
+      SentryWidget(
+        child: MyApp(),
+      ),
+    ),
   );
 
   // you can also configure SENTRY_DSN, SENTRY_RELEASE, SENTRY_DIST, and
@@ -94,7 +98,11 @@ Future<void> main() async {
         // Note: Profiling alpha is available for iOS and macOS since SDK version 7.12.0
         options.profilesSampleRate = 1.0;
       },
-      appRunner: () => runApp(MyApp()),
+      appRunner: () => runApp(
+        SentryWidget(
+          child: MyApp(),
+        ),
+      ),
     );
   } (error, stackTrace) {
     // Automatically sends errors to Sentry, no need to do any

docs/platforms/flutter/session-replay/index.mdx

@@ -27,14 +27,21 @@ dependencies:
 
 ## Setup
 
-To set up the integration, add the following to your Sentry initialization:
+To set up replay, enable it by setting a non-zero sample rate. Also, make sure you've `SentryWidget` wrapping your app:
 
 ```dart
-await SentryFlutter.init((options) {
-  ...
-  options.experimental.replay.sessionSampleRate = 1.0;
-  options.experimental.replay.onErrorSampleRate = 1.0;
-});
+await SentryFlutter.init(
+  (options) {
+    ...
+    options.experimental.replay.sessionSampleRate = 1.0;
+    options.experimental.replay.onErrorSampleRate = 1.0;
+  },
+  appRunner: () => runApp(
+    SentryWidget(
+      child: MyApp(),
+    ),
+  ),
+);
 ```
 
 ## Verify