Add LLM usage checkbox to PR template

[yeongbin05]

Description

This PR implements the suggestion discussed in #2260 to add an AI/LLM usage checkbox to the PR template.
This addition helps reviewers understand better the context of the contribution.

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

AI/LLM Usage

• This PR includes code generated with the help of an AI/LLM

[matthiask]

Thank you! I think we could follow Django's lead and implement something like this:

django/django@4f580c4

I know this isn't exactly the thing which was discussed in the linked issue. We're in an environment which is moving quickly and I have a more nuanced position now where I think it's important that the code is verified, the fact that an LLM was used per se is less interesting than I thought a few months ago. Others may disagree though.

[tim-schilling]

@matthiask For me, the concern is more about trying to provide fair feedback. If someone used an AI to generate code, there's probably parts of it that were done that weren't thought about (using a docstring block versus a line for example). It would be easier to review code when I know an LLM was used because there's less attachment to the code. I don't know. This doesn't need to be a permanent thing either, but I know right now I've made a few comments about potentially LLM generated code. Maybe I should be guiding them on how to use it better? Or how to review their own code before submitting it?

The verified bit doesn't really mean much for me. If I don't know the person, I have to verify the code myself regardless of whether they check that box.

[matthiask]

It would be easier to review code when I know an LLM was used because there's less attachment to the code. I don't know.

Ah yes, that's a good point. I was thinking more about the quality of the result, and not so much about the feelings of the submitter.

I think the pull request is good as it is then; I think people aren't discouraged by the current wording from contributing to the project, whether they have been using LLMs or not.

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.