Set workstation type to 100 for headless systems in documentation CI

Author: bowenszhu

.github/workflows/documentation.yml

@@ -20,4 +20,5 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # For authentication with GitHub Actions token
           DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }} # For authentication with SSH deploy key
+          GKSwstype: "100" # no output on headless systems
         run: julia --project=docs/ docs/make.jl