Add .gitlab templates for new work items and merge requests
It's like the description of creating a new task says:
> Add [description templates ](https://git.drupalcode.org/help/user/project/description_templates)to help your contributors communicate effectively!
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We have some examples we could use,
Issue templates:
https://git.drupalcode.org/project/ai/-/tree/1.x/.gitlab/issue_templates?ref_type=heads
Merge request templates:
https://git.drupalcode.org/project/ai/-/blob/1.x/.gitlab/merge_request_templates/Default.md?ref_type=heads
This should make contribution easier now we've moved to GitLab issues.
See original issue in `drupal/ai` https://git.drupalcode.org/project/ai/-/work_items/3586398#description
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