chore(github): add pull request template (#671)

relates to STACKITTPR-138

Author: rubenhoenle

.github/pull_request_template.md

@@ -0,0 +1,18 @@
+## 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
+- [ ] Code format was applied: `make fmt`
+- [ ] Examples were added / adjusted (see e.g. [here](https://github.com/stackitcloud/stackit-cli/blob/ef291d1683ca5b0d719ec0a26ecb999a32685117/internal/cmd/ske/cluster/create/create.go#L49-L63))
+- [ ] Docs are up-to-date: `make generate-docs`
+- [ ] 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)