Explain how to override environment names

What does this MR do?

This MR attempts to address the need identified in #211592 (closed) - https://docs.gitlab.com/ee/user/clusters/applications.html#usage needs to discuss the * scope, and describe how to override this scope.

I've patched together words from several different engineers in the hopes of coming up with something that is coherent, but it likely won't sound smooth to a more experienced user.

It creates a new subheading, Override environment or cluster scope, which contains a bare-bones YAML example scraped from @bdowney's example file (https://gitlab.com/gitops-demo/apps/cluster-management/-/blob/master/.gitlab-ci.yml). I tried to anonymize it slightly, and I took out much of the .managed-apps section, but I'm guessing here at how much the user truly needs. Treat it more as "hey code should go here" and edit it accordingly, rather than assuming I got it even remotely right.

Related issues

Closes #211592 (closed)

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