Add merge request template

Add a merge request template which aids in contributing to the codebase.

Also see https://docs.gitlab.com/ee/user/project/description_templates.html.

.gitlab/merge_request_templates/Default.md

+# Pull Request Title
+
+<!-- [Short description of the changes] -->
+
+## Description
+
+<!-- [DO NOT MODIFY] -->
+
+Wants to merge: %{source_branch} into %{target_branch}
+
+<!-- [Detailed explanation of the changes, why they are necessary, and any relevant context.] -->
+
+## Type of change
+
+<!-- [Mark the appropriate option with an "x".] -->
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Enhancement
+- [ ] Documentation update
+- [ ] Other (specify right below)
+
+## Merge request commits
+
+<!-- [DO NOT MODIFY] -->
+
+%{all_commits}
+
+## Related Issues
+
+<!-- Fixes #123, Addresses #456 -->
+
+## Screenshots or GIFs
+
+<!-- [Insert images or links for visual aid if applicable] -->
+
+## Checklist
+
+- [ ] I have tested the code with the changes manually.
+- [ ] My code follows the project's style guidelines.
+- [ ] I have documented my code for others to understand.
+- [ ] I have updated documentation as needed (including `README.md`, code
+  comments and doc strings).
+
+## Reviewer Guidelines
+
+<!-- [Provide instructions or areas to focus on during the review.] -->
+
+## Additional Notes
+
+<!-- [Add any other relevant information or special instructions.] -->
+
+## @mentions
+
+<!-- @reviewer1 @reviewer2 -->