Update Nuxt README for bundled server initialization

[nicohrubec]

Replace the generic runtime-hook externalization advice in packages/nuxt/README.md with guidance matching the default v11 setup.

Explain briefly that the SDK bundles the root-level sentry.server.config.ts, initializes Sentry automatically, and instruments dependencies at build time. Standard setup does not require a separate --import preload.

Link to the server-side initialization guide for details and troubleshooting.

Docs PR: getsentry/sentry-docs#19466

[linear-code]

JS-3740