Clarify how to handle dynamic config

[eric-burel]

See https://github.com/vercel/next.js/discussions/14308

Runtime configs are not advised, because they can't be deployed in a serverless environment. However, environment variable are clearly not sufficient to setup the app: how to parse arrays, how to dynamically set a value? If only because they are string based, and not very intuitive to consume.

See src/lib/api/cors for instance:

• we can't easily tell that cors can be set using APOLLO_SERVER_CORS_WHITELIST
• we need to parse it, since its not an array but a string
• no nesting, so except dirty names with long prefixes

[eric-burel]

Related to #62

[eric-burel]

Wrote an article about this subject: https://blog.vulcanjs.org/how-to-set-configuration-variables-in-next-js-a81505e43dad Assigned to myself until I can reproduce the same setup as in Aplines: ability to check environment variables presence + the best possible setup for dynamic config. #62 is still relevant as well, to avoid relying too much on env variables, but it's farther away from Next behaviour.