Research spike(docs): Include `environment` into CI deployment examples in our documentation

We want to increase the usage of environments/deployments features.

And we have a ton of examples of deploying somewhere in our documentation. Most of these examples don't use the environment keyword. (E.g. click on a few deployment examples in https://docs.gitlab.com/ee/ci/examples/)

If we add the environment keyword in these, more people will discover our deployment features.

(we can start by grep-ing stage: deploy in the main repository, and fixing all these cases)