[Pages] Include documentation for Node Version in nextjs/deploy-a-static-nextjs-site.mdx

[schemburkar]

Existing documentation URL(s)

• https://developers.cloudflare.com/pages/framework-guides/nextjs/deploy-a-static-nextjs-site/

What changes are you suggesting?

Recent Next.js versions require a minimum Node.js version to be used in builds.
Most other documentation on specifying Node.js version applies to a full Next.js site.

For a static website, the simplest approach is to update the .nvmrc file as Cloudflare builds use nvm for detection of node version

Additional information