Explicit fallback to maintainers

We can use this action to make it explicit when something isn't covered by folks in the CODEOWNER:

- name: Run Codeowners merge check
  uses: OSS-Docs-Tools/code-owner-self-merge@v1
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
  with:
    merge_method: 'squash'
    if_no_maintainers_add_label: 'maintainers'
    if_no_maintainers_assign: '@orta @sandersn'