Set Plausible domain for production website

[vgeorge]

In order to start collecting analytics from the production website, it is necessary to add the following environment variable to the AWS deployment environment:

NEXT_PUBLIC_PLAUSIBLE_DOMAIN=food.theplotline.org

In PR #53, I mentioned Github Secrets, but that is not the correct method for setting this configuration.

@cameronkruse, would you be able to update the production environment?

cc @wrynearson