Establish guidelines for usage of AI in PRs (JabRef#11940)

* Add check for AI-generated PRs

* Refine

* Add to greeting

* Remove reference to code of conduct

* Change mandatory check

* Refine condition

Co-authored-by: Ruslan <ruslanpopov1512@gmail.com>
Co-authored-by: Subhramit Basu Bhowmick <74734844+subhramit@users.noreply.github.com>
Co-authored-by: Carl Christian Snethlage <50491877+calixtus@users.noreply.github.com>
Co-authored-by: ThiloteE <73715071+ThiloteE@users.noreply.github.com>

* Discard changes to .github/workflows/add-greeting-to-issue.yml

* Update CONTRIBUTING.md

Co-authored-by: Ruslan <ruslanpopov1512@gmail.com>
Co-authored-by: Subhramit Basu Bhowmick <74734844+subhramit@users.noreply.github.com>
Co-authored-by: Carl Christian Snethlage <50491877+calixtus@users.noreply.github.com>
Co-authored-by: ThiloteE <73715071+ThiloteE@users.noreply.github.com>

* Update PULL_REQUEST_TEMPLATE.md

Co-authored-by: Ruslan <ruslanpopov1512@gmail.com>
Co-authored-by: Subhramit Basu Bhowmick <74734844+subhramit@users.noreply.github.com>
Co-authored-by: Carl Christian Snethlage <50491877+calixtus@users.noreply.github.com>
Co-authored-by: ThiloteE <73715071+ThiloteE@users.noreply.github.com>

---------

Co-authored-by: Oliver Kopp <kopp.dev@gmail.com>
Co-authored-by: Ruslan <ruslanpopov1512@gmail.com>
Co-authored-by: Carl Christian Snethlage <50491877+calixtus@users.noreply.github.com>
Co-authored-by: ThiloteE <73715071+ThiloteE@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,6 +13,7 @@ Don't reference an issue in the PR title because GitHub does not support auto-li
 - [x] done; [ ] not done / not applicable
 -->
 
+- [x] I own the copyright of the code submitted and I licence it under the [MIT license](https://github.com/JabRef/jabref/blob/main/LICENSE)
 - [ ] Change in `CHANGELOG.md` described in a way that is understandable for the average user (if applicable)
 - [ ] Tests created for changes (if applicable)
 - [ ] Manually tested changed features in running JabRef (always required)

CONTRIBUTING.md

@@ -113,6 +113,17 @@ We also show all contributors in our blog posts. See [Release 5.15 blog post](ht
 
 Your contribution is considered being made under [MIT license](https://tldrlegal.com/license/mit-license).
 
+#### Notes on AI usage
+
+Please keep these two principles in mind when you contribute:
+
+1. Never let an LLM speak for you.
+2. Never let an LLM think for you.
+
+More reading on that is available at <https://roe.dev/blog/using-ai-in-open-source>.
+
+We reserve the right to reject pull requests that contain little or no genuine and original contribution from the contributor.
+
 ### Development hints
 
 #### When adding an external dependency