.github: Add PR template

.github/pull_request_template.md

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+## Description
+
+<!-- Enter a description of what your pull request does. -->
+
+## AI Usage
+
+<!-- Select which box below describes your use of generative AI for this PR. -->
+
+- [ ] I **did not** use generative AI at all in making the content of this pull
+  request.
+- [ ] I **did** use generative AI in some form in making the content of this
+  pull request. I have described my use of AI below.
+
+<!-- If you used generative AI, you must briefly describe how you used it, e.g.
+you copied output directly from an LLM, you used AI to draft code/documentation
+but made significant manual edits, etc.-->
+
+## Contributor Checklist
+
+<!-- Please review the checklist below and attest by checking all boxes.-->
+
+- [ ] I **have manually reviewed all content** submitted to gittuf in this pull
+  request.
+- [ ] I fully understand the content I am submitting.
+- [ ] The changes introduced are documented and have tests included if
+  applicable.
+- [ ] My changes do not infringe on copyright/trademarks/etc.
+- [ ] All commits in this pull request include a [DCO
+  Signoff](https://wiki.linuxfoundation.org/dco).
+- [ ] By submitting this pull request, I agree to follow the gittuf [Code of
+  Conduct](https://github.com/gittuf/community/blob/main/CODE-OF-CONDUCT.md).