Add Whats New for 15.3

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds "What's New" content for 15.3 per this process.

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How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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