chore(github): add pull request template (#808)

relates to STACKITTPR-138

Author: rubenhoenle

.github/pull_request_template.md

@@ -0,0 +1,17 @@
+## Description
+
+<!-- **Please link some issue here describing what you are trying to achieve.**
+
+In case there is no issue present for your PR, please consider creating one.
+At least please give us some description what you are trying to achieve and why your change is needed. -->
+
+relates to #1234
+
+## Checklist
+
+- [ ] Issue was linked above
+- [ ] **No generated code was adjusted manually** (check [comments in file header](https://github.com/stackitcloud/stackit-sdk-python/blob/main/services/dns/src/stackit/dns/api_client.py#L10-L12))
+- [ ] Examples were added / adjusted (see `examples/` directory)
+- [ ] Unit tests got implemented or updated
+- [x] Unit tests are passing: `make test` (will be checked by CI)
+- [x] No linter issues: `make lint` (will be checked by CI)