Add some better docs to signpost that you shouldn't run `ember serve --environment=production`

I recently had a conversation in Discord where it became obvious that the "insider knowledge" that ember-cli's server isn't fit for production use wasn't readily apparent.

I asked where would be good places to make this explicit in the docs and there are two recommendations:

- https://github.com/ember-fastboot/ember-cli-fastboot#fastboot-app-server-configuration
- https://github.com/ember-fastboot/ember-cli-fastboot#running

that last example is particularly bad because it hints that you might use ember-cli for something "serious" 🙈

We need to make this as obvious as possible, even going so far to put it as a **WARNING** as visually large as possible.