add docs for sentryHandleRequest (#13122)

Author: chargome

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

@@ -188,6 +188,22 @@ export const handleError: HandleErrorFunction = (error, { request }) => {
     console.error(error);
   }
 };
+
+- export default function handleRequest(
++ function handleRequest(
+  request: Request,
+  responseStatusCode: number,
+  responseHeaders: Headers,
+  routerContext: EntryContext,
+  loadContext: AppLoadContext,
+) {
+  return new Promise((resolve, reject) => {
+    // ...
+  }
+}
+
++ export default Sentry.sentryHandleRequest(handleRequest);
+
 // ... rest of your server entry
 ```