Examples: ensure the readiness of Nuxt
[issue link]The examples mounts the middleware without waiting for readiness:
This applies to our own commands as well.
https://github.com/nuxt/nuxt.js/blob/d1d637f0c4a55fb68895740ceb97b0e92e07fafd/bin/nuxt-start#L54-L83
Although in practice this have never caused an issue, we should document the fact that Nuxt itself is initialized asynchronously, and ensure the readiness before actually starting the server.