Upload docs.

Author: samcunliffe

Diff for: .github/workflows/docs.yml

@@ -38,7 +38,7 @@ jobs:
           working-directory: ./docs
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: docs
           path: docs/_build