docs(flutter): Add Rewrite Frames documentation (#10470)

* docs(flutter): Add  documentation

* style updates

docs/platforms/flutter/configuration/draining.mdx

@@ -1,6 +1,6 @@
 ---
 title: Shutdown and Draining
-sidebar_order: 70
+sidebar_order: 80
 description: "Learn more about the default behavior of our SDK if the application shuts down unexpectedly."
 ---
 

docs/platforms/flutter/configuration/rewriteframes.mdx

@@ -0,0 +1,32 @@
+---
+title: Rewrite Frames
+sidebar_order: 70
+description: "Learn how and why to rewrite stack trace frames in Sentry's flutter SDK."
+---
+
+There are times when you may want to rewrite stack trace frames before sending them to Sentry. For example, if you're publishing your Flutter application on the web and want to set a `url_prefix` because it's not deployed at the root of your URL. In this case, the absolute path of your stack frames also needs to include the same prefix so that the source maps can be found for deobfuscation. 
+
+To set the `url_prefix`, use the [Sentry Dart Plugin](https://github.com/getsentry/sentry-dart-plugin). Below is an example of how to update the stack frame's absolute path to include the prefix using the <PlatformLink to="/configuration/options/#before-send"><PlatformIdentifier name="before-send" /></PlatformLink> hook.
+
+```dart
+options.beforeSend = (event, hint) async {
+  final exceptions = event.exceptions?.map((exception) {
+    final stackTrace = exception.stackTrace;
+    if (stackTrace != null) {
+      final frames = stackTrace.frames.map((frame) {
+        const baseUrl = 'https://example.com/';
+        final modifiedAbsPath = frame.absPath?.replaceFirst(
+          baseUrl,
+          '${baseUrl}my_prefix/',
+        );
+        return frame.copyWith(absPath: modifiedAbsPath);
+      }).toList();
+      return exception.copyWith(
+        stackTrace: SentryStackTrace(frames: frames),
+      );
+    }
+    return exception;
+  }).toList();
+  return event.copyWith(exceptions: exceptions ?? []);
+};
+```