chore: add a GitHub PR template

Includes a checklist of things to be verified by authors and reviewers.

Fixes: MODELIX-796

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+TODO description of the PR
+
+## To be verified by reviewers
+
+* [ ] Relevant public API members have been documented
+* [ ] Documentation related to this PR is complete
+  * [ ] Boundary conditions are documented
+  * [ ] Exceptions are documented
+  * [ ] Nullability is documented if used
+* [ ] Touched existing code has been extended with documentation if missing
+* [ ] Code is readable
+* [ ] New features and fixed bugs are covered by tests