Add pull request template for AI usage disclosure

[preetk25627-dotcom]

Summary

This PR adds a pull request template with an optional AI usage disclosure section.

It implements the lightweight approach discussed in #1919 by allowing contributors to indicate whether AI tools were used while preparing the pull request and, if applicable, briefly describe how they were used and what was manually reviewed.

PR intentionally focuses on the pull request template and does not introduce broader AI contribution policy documentation.
Related to #1919

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[cantel]

Hi @preetk25627-dotcom , thanks a lot for picking up on the issue, studying the conversations and proactively putting forth one of the concrete suggestions. I'm just busy writing a post on thoughts on the issues page, and then we can get back to this.

[cantel]

Let's see what the general opinion is. Perhaps what people might want to comment on here is whether we just want a tick box or also ask for the additional fields.
In addition why not another tick box: "I certify I have verified and understood the changes this PR implements."
Or, if we have a contributing file stating some basic rules (incl. disclose your AI usage), a tick box to say something like "I acknowledge the basic rules for contributors".