docs(tanstackstart): Add alpha notice (#14380)

## DESCRIBE YOUR PR
Add alpha notice and information about not auto-capturing errors on the
server-side.

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- [ ] Other deadline: <!-- ENTER DATE HERE -->
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## SLA

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## 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)

Author: s1gr1d

docs/platforms/javascript/guides/tanstackstart-react/index.mdx

@@ -3,6 +3,17 @@ title: TanStack Start React
 description: "Learn how to set up and configure Sentry in your TanStack Start React application, capturing your first errors, and viewing them in Sentry."
 ---
 
+<Alert>
+
+  This SDK is currently in **ALPHA**. Alpha features are still in progress, may have bugs and might include breaking changes.
+  Please reach out on  [GitHub](https://github.com/getsentry/sentry-javascript/issues/new/choose) if you have any feedback or concerns.
+  This SDK is for [TanStack Start (React)](https://tanstack.com/start/latest/docs/framework/react/overview), for
+  using [TanStack Router (React)](https://tanstack.com/router/latest/docs/framework/react/overview) see our React docs for
+  [TanStack Router](/platforms/javascript/guides/react/features/tanstack-router).
+
+</Alert>
+
+
 <PlatformContent includePath="getting-started-prerequisites" />
 
 ## Step 1: Install
@@ -44,11 +55,11 @@ const router = createRouter();
 
 Sentry.init({
   dsn: "___PUBLIC_DSN___",
-  
+
   // Adds request headers and IP for users, for more info visit:
   // https://docs.sentry.io/platforms/javascript/guides/tanstackstart-react/configuration/options/#sendDefaultPii
   sendDefaultPii: true,
-  
+
   integrations: [
     // ___PRODUCT_OPTION_START___ performance
     Sentry.tanstackRouterBrowserTracingIntegration(router),
@@ -96,7 +107,7 @@ import * as Sentry from "@sentry/tanstackstart-react";
 
 Sentry.init({
   dsn: "___PUBLIC_DSN___",
-  
+
   // Adds request headers and IP for users, for more info visit:
   // https://docs.sentry.io/platforms/javascript/guides/tanstackstart-react/configuration/options/#sendDefaultPii
   sendDefaultPii: true,
@@ -171,6 +182,10 @@ registerGlobalMiddleware({
 
 ### Capture Errors in your Error Boundaries and Components
 
+<Alert level="info">
+  Automatic error monitoring is not yet supported on the on the server-side of TanStack Start. Use `captureException` to manually capture errors in your server-side code.
+</Alert>
+
 The Sentry SDK cannot capture errors that you manually caught yourself with error boundaries.
 
 If you have React error boundaries in your app and you want to report errors that these boundaries catch to Sentry, apply the [`withErrorBoundary` wrapper](/platforms/javascript/guides/react/features/error-boundary/) to your `ErrorBoundary`: