Document simplest build, test, deploy case, in a single job

There's a lot of confusion about having build, test, deploy happen across stages, and thus across runners, but not automatically sharing context between jobs. We've got some documentation how to share that context using artifacts, but really, a lot of people just want all three steps to run on the same runner. This can easily be achieved by putting all of your tasks in a single job. Let's document this.

Inspired by https://about.gitlab.com/2016/07/29/the-basics-of-gitlab-ci/#comment-3310499980 and others.

/cc @bikebilly @axil