chore: add merge request template for gitlab

.gitlab/merge_request_templates/default.md

+## Summarize the changes
+
+%{all_commits}
+
+## Ensure that
+
+- [ ] New code is covered by unit and integration tests.
+- [ ] Related issues would be automatically closed with
+      gitlab's closing pattern (`Closes #1, #2`).
+- [ ] Public modules are documented (check the rendered version with
+      `cargo doc --open`).
+- [ ] (if PEST grammar is changed) EBNF grammar reflects these changes
+      (check the result with railroad diagram [generator].
+
+## Next steps
+
+- Update sbroad submodule in [picodata/picodata].
+- (if EBNF grammar is changed) create a follow-up issue in [picodata/docs].
+
+[generator]: https://bottlecaps.de/rr/ui
+[picodata/docs]: https://git.picodata.io/picodata/picodata/docs/-/issues/new
+[picodata/picodata]: https://git.picodata.io/picodata/picodata/picodata