Docs: Move 'generate test vulns' topic to dev docs

What does this MR do?

I've come across this page a few times and I keep thinking it's a bit of a mismatch for the user-focused part of the docs. Asking people to clone the GitLab repo and run scripts against the API seems like not the procedure we'd want to give people who want to test vulns—rather we should be telling them to scan their own projects, or optionally giving them specific demo projects that we recommend scanning.

If this merges, we'll need to follow up with a nav-tree update in gitlab-org/gitlab-docs.

Related issues

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