dev: Improve the pull request template

Author: marienfressinaud

.github/pull_request_template.md

@@ -1,8 +1,25 @@
-Pull request checklist:
+## Related issue(s)
 
-- [ ] code is manually tested
-- [ ] tests are updated
-- [ ] documentation is updated (including comments, commit messages, migration notes, …)
+<!-- Copy-paste the URL to the related issue(s) if any ("N/A" if not applicable).
+  -->
 
-_If you think one of the item isn’t applicable to the PR, please check it
-anyway and precise `N/A` next to it._
+## How to test manually
+
+<!-- List actions (step by step) of what have to be done in order to test your
+  -- changes manually ("N/A" if not applicable).
+  -->
+
+## Reviewer checklist
+
+<!-- Please don't change or remove this checklist. It will be used by the
+  -- reviewer to make sure to not forget important things.
+  -- Reviewer: if you think one of the item isn’t applicable to this PR,
+  -- please check it anyway.
+  -->
+
+- [ ] Code is manually tested
+- [ ] Translations are synchronized
+- [ ] Tests are up to date
+- [ ] Copyright notices are up to date
+- [ ] Documentation is up to date
+- [ ] Pull request has been reviewed and approved