docs(tanstackstart-react): Add tanstackstart router feature page to tanstackstart-react docs (#15659)

Adding a feature page to the TanstackStart-React SDK docs for the
TanstackStart router integration ([a similar
page](https://docs.sentry.io/platforms/javascript/guides/react/features/tanstack-router/)
already exists for React, adapted it for Tanstack).

Author: nicohrubec

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

@@ -0,0 +1,10 @@
+---
+title: TanStack Start React Features
+description: "Learn how Sentry's TanStack Start React SDK exposes custom components for first class integration with the TanStack Start React framework."
+sidebar_order: 4
+---
+
+The Sentry TanStack Start React SDK offers TanStack Start-specific features for first class integration with the framework.
+
+<PageGrid />
+

docs/platforms/javascript/guides/tanstackstart-react/features/tanstack-router.mdx

@@ -0,0 +1,48 @@
+---
+title: TanStack Router
+description: "Learn about Sentry's TanStack Router integration."
+---
+
+The TanStack Router integration is included in the `@sentry/tanstackstart-react` package and is compatible with version `1.64.0` of `@tanstack/react-router` and above.
+
+<Alert title="Note">
+
+The TanStack Router integration is designed to work with Sentry Tracing. Please see <PlatformLink to="/tracing/#enable-tracing">Getting Started with TanStack Start React Performance</PlatformLink> for more details on how to set up and install the SDK.
+
+</Alert>
+
+The TanStack Router instrumentation uses your TanStack Router routes to create `pageload/navigation` transactions to ensure you collect meaningful performance data about the health of your page loads and associated requests.
+
+## Usage
+
+To use the TanStack Router integration, pass the `Sentry.tanstackRouterBrowserTracingIntegration` inside your `integrations` option:
+
+```tsx {diff} {filename:src/router.tsx}
+import * as Sentry from "@sentry/tanstackstart-react";
+import { createRouter } from '@tanstack/react-router'
+
+// Create a new router instance
+export const getRouter = () => {
+  const router = createRouter();
+
+  if (!router.isServer) {
+    Sentry.init({
+      dsn: "___PUBLIC_DSN___",
+      integrations: [Sentry.tanstackRouterBrowserTracingIntegration(router)],
+
+      // Setting a sample rate is required for sending performance data.
+      // We recommend adjusting this value in production, or using tracesSampler
+      // for finer control.
+      tracesSampleRate: 1.0,
+    });
+  }
+  
+  return router;
+}
+```
+
+## Next Steps:
+
+- [Return to **Getting Started**](../../)
+- [Return to the main integrations page](../)
+