chore(github): add pull request template (#1813)

relates to STACKITTPR-138

Author: rubenhoenle

.github/pull_request_template.md

@@ -0,0 +1,21 @@
+## 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-go/blob/783b7fa78e41d072a3ff08e246a13f1bef5aa764/services/dns/api_default.go#L9))
+- [ ] Changelogs
+    - [ ] Changelog in the root directory was adjusted (see [here](https://github.com/stackitcloud/stackit-sdk-go/blob/f1e375a38064f798821d22a951bc74ca8a9c8845/CHANGELOG.md))
+    - [ ] Changelog(s) of the service(s) were adjusted (see e.g. [here](https://github.com/stackitcloud/stackit-sdk-go/blob/f1e375a38064f798821d22a951bc74ca8a9c8845/services/dns/CHANGELOG.md))
+- [ ] Code format was applied: `make fmt`
+- [ ] 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)