gitlab(picodata): add merge request template

NO_DOC=chore
NO_CHANGELOG=chore
NO_TEST=chore

.gitlab/merge_request_templates/default.md

+Summary
+%{all_commits}
+
+Close #...
+Docs follow-up: not necessary / [new issue](https://git.picodata.io/picodata/tarantool-doc/-/issues/new)