Update details on secure templates with MR pipelines

What does this MR do?

Moves and clarifies the details added in !46565 (merged).

I think that users who are having trouble with the secure templates will likely look at the secure feature docs first, to try to figure out the problem. From there, we can crosslink back to the MR pipelines docs if they need more details. I'm not sure as many people will look at the MR pipeline docs first, for Secure template trouble.

The example config was incorrect as well, so I updated that too.

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