Document node version and Next.js requirements

[revodavid]

This issue is for a: (mark with an x)

- [ ] documentation issue or request

Minimal steps to reproduce

npm install nodejs npm
git clone https://github.com/Azure-Samples/contoso-web
npm install next

Any log messages given by the failure

Attempting npm run dev with a version of Node.js 12 or lower (the default on some distros) will create the confusing error:

SyntaxError: Unexpected token '?'

Expected/desired behavior

npm run dev works

OS and Version?

CodeSpace for contoso-chat

Mention any other details that might be useful

Tested with Node 20 (current LTS) and it works. It would be useful to give instructions on how to install Node 20 and Next.js