Helm documentation for ImagePullSecrets less confusing

What does this MR do?

Modify the documentation to make it less confusing with ImagePullSecrets.
It now shows the newest version first with an example and talk about the legacy one afterwards.

Related issues

#31066 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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Reviewer's checklist

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