docs: Update custom 404.html for VitePress docs

See: #1249

Author: matz3

.github/workflows/deploy-vitepress-docs.yaml

@@ -87,8 +87,8 @@ jobs:
                   # Symlink the api docs to avoid duplication
                   ln -s ../${DOC_VERSION}/api ./gh-pages/${DOC_ALIAS}/api
 
-                  # TODO: Enable when v5 release is done
-                  # cp internal/documentation/scripts/resources/custom404.html ./gh-pages/404.html
+                  # Custom 404 page
+                  cp internal/documentation/scripts/resources/custom404.html ./gh-pages/404.html
             - name: Publish Docs
               run: |
                   cd ./gh-pages