chore: require copyright assignment in PR template (#816)

Author: szhorvat

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+<!-- Please disclose any use of LLMs (ChatGPT, Copilot, Gemini, etc.) during the preparation of this PR. -->
+
+<!-- The following text must be left intact and the box checked before the PR can be merged. -->
+
+- [ ] By submitting this pull request, I assign the copyright of my contribution to _The igraph development team_.