fix(apple): Docs for enableMetricKitAttachments

Add docs for enableMetricKitAttachments available on Cocoa 8.29.0 and
above.

docs/platforms/apple/common/configuration/metric-kit.mdx

@@ -34,3 +34,23 @@ SentrySDK.start { options in
     options.enableMetricKit = YES;
 }];
 ```
+
+You can enable `enableMetricKitAttachments` to view the raw MetricKit diagnostic payload in JSON format as an attachment on the converted event in Sentry. This feature is available on Cocoa 8.29.0 and up.
+
+```swift {tabTitle:Swift}
+import Sentry
+
+SentrySDK.start { options in
+    options.dsn = "___PUBLIC_DSN___"
+    options.enableMetricKitAttachments = true
+}
+```
+
+```objc {tabTitle:Objective-C}
+@import Sentry;
+
+[SentrySDK startWithConfigureOptions:^(SentryOptions *options) {
+    options.dsn = @"___PUBLIC_DSN___";
+    options.enableMetricKitAttachments = YES;
+}];
+```