Haddock: Add MR template for Haddock

.gitlab/merge_request_templates/Haddock.md

+Thank your for your contribution to Haddock!
+
+  * [ ] This MR relates to \<ticket number>
+  * [ ] I have read the [CONTRIBUTING](./utils/haddock/CONTRIBUTING.md) guide
+  * [ ] I have inserted a CHANGELOG entry if this warrants it
+  * [ ] I have squashed my commits
+  * [ ] I have added tests if necessary
+
+  * [ ] I have updated the documentation
+
+If you have any questions don't hesitate to open your merge request and inquire
+in a comment. If your patch isn't quite done yet please do add prefix your MR
+title with `Draft:`.
+
+Once your change is ready please remove the `Draft:` tag and wait for review. If
+no one has offered a review in a few days then please leave a comment mentioning
+@triagers and apply the ~"Blocked on Review" label.