feat(apple): Add iOS App Hang rate docs (#14944)

## DESCRIBE YOUR PR
This PR adds a new "App Hang Rate" section to the iOS App Hangs
documentation (`platforms/apple/configuration/app-hangs/`). It clarifies
the definition of App Hang Rate for Apple platforms, emphasizing that
the rate is calculated *only* for **Fatal App Hangs**. The new section
also explains how to view this metric in Sentry and distinguishes
between fatal and non-fatal app hangs in the context of rate
calculation.

Closes #14815

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---------

Co-authored-by: Cursor Agent <[email protected]>

Author: romtsn

docs/platforms/apple/common/configuration/app-hangs.mdx

@@ -184,3 +184,48 @@ SentrySDK.start { options in
     options.enableReportNonFullyBlockingAppHangs = NO;
 }];
 ```
+
+## App Hang Rate
+
+<Alert level="info">
+
+The SDK supports App Hang Rate calculation starting with version 8.50.0. Additionally, you have to enable App Hangs V2 via `options.enableAppHangTrackingV2 = true` in your SDK configuration.
+
+</Alert>
+
+App hangs occur when the main UI thread of an application becomes unresponsive. When configured, Sentry's Apple SDK reports app hang events, and Sentry calculates an App Hang Rate based on these events and user sessions.
+
+App Hang Rate is the percentage of unique users who have experienced a **Fatal App Hang** during the specified time period.
+
+<Alert level="info">
+
+The App Hang Rate in Sentry only accounts for **Fatal App Hangs** - app hangs that lead to your app being terminated, either by the user or the OS watchdog. Non-fatal app hangs are captured as events but do not contribute to the App Hang Rate calculation.
+
+</Alert>
+
+### Fatal App Hangs
+
+Fatal app hangs are the most critical type of app hang because they result in app termination and directly impact user experience. These occur when:
+
+- The user manually force-quits the app during a hang
+- The system watchdog terminates the app due to unresponsiveness
+
+Fatal app hangs are distinguished by their `exception.type`:
+
+- `Fatal App Hang Fully Blocked`: A completely blocking app hang that leads to termination
+- `Fatal App Hang Non Fully Blocked`: A partially blocking app hang that leads to termination
+
+### Viewing Your App Hang Rate
+
+To see your App Hang Rate in Sentry:
+
+1. Go to the **Projects** page
+2. Click on your Apple project
+3. Look for the "App Hang Rate" card in the "Project Details" section
+4. Click "View Issues" to see a list of app hang issues that were created in the selected timeframe and environment
+
+The App Hang Rate card displays the percentage of users affected by fatal app hangs, helping you understand the impact on your user base and prioritize performance improvements.
+
+![App Hang Rate](./img/app-hang-rate.png)
+
+Since App Hang Rate only includes fatal app hangs, it represents the most severe hangs that directly affect user retention and app store ratings.