Fix git unmodified check in CI (#409)

the diff-index picks up changes if files have been touched

Author: maxammann

.github/workflows/generate-docs.yml

@@ -46,7 +46,7 @@ jobs:
 
       - name: Check for modified files
         id: git-check
-        run: echo ::set-output name=modified::$(if git diff-index --ignore-submodules --quiet HEAD --; then echo "false"; else echo "true"; fi)
+        run: echo "modified=$(if [[ -z $(git status --porcelain --untracked-files=no --ignore-submodules) ]]; then echo "false"; else echo "true"; fi)" >> "$GITHUB_OUTPUT"
 
       - name: Commit changes, if any
         if: steps.git-check.outputs.modified == 'true'