Add robots.txt at the root of dev and stage domains

All dev and stage sites have a robots.txt file that prohibits bots from indexing these websites. This is configured at planet4-robots.

eg. https://www-dev.greenpeace.org/international/robots.txt

Despite that, some of these website still poping up on search results:
https://www.google.com/search?q=www-dev.greenpeace.org

This can be the result of a missing robots.txt file from the root of the www-dev domain. We could utilize a Cloudflare worker that could just handle this request.