Indicate file where to place the code fragments

What does this MR do?

When reading this documentation page, I was not able to figure out what file this was to be added to. In hindsight, it is relatively obvious, but really could be more direct and clear.

To be confident, I had to read some other docs and eventually found: https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/e2779d446345f90c81da071e07f922ab3da5530e/doc/kubernetes_ci_access.md#apiv4joballowed_agents-api

Related issues

I didn't find one when I searched, and I didn't create one. Let me know if I need to.

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