Draft: Add index for design docs for prompt registry

What does this MR do and why?

NOTE: This is currently a proposal only. It is subject to legal approval.

Add design blueprint for prompt registry

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

solves #432594 (closed)