Add a codeowners file to gitlab-docs

It was really bugging me that I kept getting pinged with a todo every time an MR was opened, as I was listed as a potential approver. I never understood why this didn't happen in the gitlab project, but it happens here. I finally realized that it was due to the manually input "default approvers" in the project settings, which forces us to get pinged.

Adding a simple codeowners file, which will use the docs-team for the approvers list, (and then removing the default approvers from the settings), should stop us from being pinged so much.