Add default MR template
This MR adds the default MR template similar to the AIGW one - https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/.gitlab/merge_request_templates/Default.md?ref_type=heads&plain=1
This MR adds the default MR template similar to the AIGW one - https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/.gitlab/merge_request_templates/Default.md?ref_type=heads&plain=1