Add default Next.js event environment (#3519)

This PR adds the corresponding docs for getsentry/sentry-javascript#3495, where the `process.env.NODE_ENV` is added as the `environment` by default.

Author: iker-barriocanal

src/includes/getting-started-config/javascript.nextjs.mdx

@@ -28,3 +28,5 @@ const handler = async (req, res) => {
 
 export default withSentry(handler);
 ```
+
+By default, the SDK sets the environment for events to `process.env.NODE_ENV`, although you can [set your own](/platforms/javascript/guides/nextjs/configuration/environments/).