Create a Merge Request template

To make GitLab UI's development workflow a bit easier to grasp, let's create a Merge Request template containing a few tips and reminders on how to make sure your changes can safely be merged.

Proposal

## What does this MR do?

<!--

Describe in detail what your merge request does and why.

Please keep this description up-to-date with any discussion that takes
place so that reviewers can understand your intent. This is especially
important if they didn't participate in the discussion.

-->

## Does this MR meet the acceptance criteria?

### Conformity

- [ ] [Code review guidelines](https://docs.gitlab.com/ee/development/code_review.html).
- [ ] [GitLab UI's contributing guidlines](https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/CONTRIBUTING.md).
- [ ] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a ~UX designer.
- [ ] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
- [ ] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the `@gitlab/ui` package can be upgraded quickly after the changes are released:
   - [ ] **[GitLab](https://gitlab.com/gitlab-org/gitlab)**: mr_url
   - [ ] **[Customers Portal](https://gitlab.com/gitlab-org/customers-gitlab-com)**: mr_url
   - [ ] **[Status Page](https://gitlab.com/gitlab-org/status-page)**: mr_url
- [ ] Added the `~"component:*"` label(s) if applicable.