RN: Capture Startup Crashes (#16170)

<!-- Use this checklist to make sure your PR is ready for merge. You may
delete any sections you don't need. -->

## DESCRIBE YOUR PR
*Tell us what you're changing and why. If your PR **resolves an issue**,
please link it so it closes automatically.*

## IS YOUR CHANGE URGENT?  

Help us prioritize incoming PRs by letting us know when the change needs
to go live.
- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [x] None: Not urgent, can wait up to 1 week+

After getsentry/sentry-react-native#5582 is
released

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

## LEGAL BOILERPLATE

<!-- Sentry employees and contractors can delete or ignore this section.
-->

Look, I get it. The entity doing business as "Sentry" was incorporated
in the State of Delaware in 2015 as Functional Software, Inc. and is
gonna need some rights from me in order to utilize my contributions in
this here PR. So here's the deal: I retain all rights, title and
interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my
contributions, under Sentry's choice of terms.

## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

Author: antonis

docs/platforms/react-native/manual-setup/app-start-error-capture.mdx

@@ -0,0 +1,106 @@
+---
+title: Capture App Start Errors
+sidebar_order: 50
+description: "Learn how to capture app start errors and crashes that occur before JavaScript loads using native initialization."
+---
+
+<Alert level="warning" title="Alpha Testing">
+
+Sentry React Native SDK v8 is currently in alpha testing. This feature is available for early adopters and may undergo changes before the stable release.
+
+</Alert>
+
+By default, the React Native SDK initializes the native SDK underneath the `init` method called on the JS layer. As a result, the SDK has a current limitation of not capturing native crashes that occur prior to the `init` method being called on the JS layer.
+
+Starting with SDK version 8.0.0, you can initialize Sentry natively before JavaScript loads, enabling capture of app start errors and crashes that occur during:
+
+- Native module initialization
+- JavaScript bundle loading
+- Early React Native bridge setup
+
+This feature uses a `sentry.options.json` configuration file and native initialization APIs that read from this file.
+
+<Alert level="info" title="SDK Version Requirement">
+
+This feature requires Sentry React Native SDK version 8.0.0 or higher.
+
+</Alert>
+
+## Configuration File
+
+Create a `sentry.options.json` file in your React Native project root with the same options you currently have in `Sentry.init`:
+
+```json {filename:sentry.options.json}
+{
+  "dsn": "https://key@example.io/value",
+  "debug": true,
+  "environment": "production",
+  "tracesSampleRate": 1.0,
+  "enableTracing": true
+}
+```
+
+<Alert level="info" title="Options Merging">
+
+Options from `sentry.options.json` are merged with options from `Sentry.init()` in JavaScript. Options specified in JavaScript take precedence over the configuration file, allowing you to override settings at runtime.
+
+</Alert>
+
+## Android Setup
+
+Initialize Sentry in your `MainApplication` class:
+
+```kotlin {filename:android/app/src/main/java/.../MainApplication.kt}
+import io.sentry.react.RNSentrySDK
+
+class MainApplication : Application(), ReactApplication {
+    override fun onCreate() {
+        super.onCreate()
+        RNSentrySDK.init(this)
+        // ... rest of your initialization code
+    }
+}
+```
+
+## iOS Setup
+
+Initialize Sentry in your `AppDelegate`:
+
+```objective-c {filename:ios/YourApp/AppDelegate.mm}
+#import <RNSentry/RNSentry.h>
+
+@implementation AppDelegate
+
+- (BOOL)application:(UIApplication *)application
+    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
+{
+    [RNSentrySDK start];
+    return [super application:application didFinishLaunchingWithOptions:launchOptions];
+}
+
+@end
+```
+
+## Expo Setup
+
+If you're using Expo, you can enable native initialization automatically using the Expo plugin:
+
+```json {filename:app.json}
+{
+  "expo": {
+    "plugins": [
+      [
+        "@sentry/react-native/expo",
+        {
+          "useNativeInit": true
+        }
+      ]
+    ]
+  }
+}
+```
+
+When `useNativeInit` is set to `true`, the Expo plugin automatically:
+- Creates `sentry.options.json` from your Expo config
+- Adds `RNSentrySDK.init()` to your Android `MainApplication`
+- Adds `RNSentrySDK.start()` to your iOS `AppDelegate`

docs/platforms/react-native/manual-setup/native-init.mdx

@@ -6,8 +6,13 @@ description: "Learn how to manually initialize the native SDKs."
 
 By default, the React Native SDK initializes the native SDK underneath the `init` method called on the JS layer. As a result, the SDK has a current limitation of not capturing native crashes that occur prior to the `init` method being called on the JS layer. You can initialize the native SDKs yourself to overcome this limitation or if you want to provide custom options above what the React Native SDK currently provides.
 
-To do this, set [autoInitializeNativeSdk](/platforms/react-native/configuration/options/#autoInitializeNativeSdk) to `false` in the init options:
+<Alert level="info" title="Capture App Start Errors">
+
+If you're using Sentry React Native SDK version 8.0.0 or higher, see the [Capture App Start Errors](/platforms/react-native/manual-setup/app-start-error-capture/) guide for a simpler approach using `sentry.options.json` and native initialization APIs.
 
+</Alert>
+
+To do this, set [autoInitializeNativeSdk](/platforms/react-native/configuration/options/#autoInitializeNativeSdk) to `false` in the init options:
 
 ```javascript
 Sentry.init({