docs: make merge request template

.gitlab/merge_request_templates/source-code-merge-request.md

+## What are the key features of this change
+
+closes:
+
+### Size of change: (small/medium/big)
+
+## Checklist
+- [ ] Javadoc is added to methods and classes
+- [ ] Tests are added to feature
+- [ ] Build tool test passed