Add UX debt label to the contribution guidelines

What does this MR do?

Adds the UX debt label to the contribution guidelines.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-design/issues/29

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-design/issues/29