Update link to documentation in GitLab readme

[HappyTetrahedron]

Checklist

• The PR has a meaningful title. It will be used to auto-generate the
  changelog.
  The PR has a meaningful description that sums up the change. It will be
  linked in the changelog.
• PR contains a single logical change (to build a better changelog).
• Update the documentation.
• Categorize the PR by adding one of the following labels so it shows up
  in the correct changelog section:
  bug, enhancement, documentation, change, breaking, dependency, internal