ci: Update checkout action to v4

Author: QuLogic

.github/workflows/docs.yml

@@ -14,7 +14,7 @@ jobs:
             github.event_name == 'push' && github.ref == 'refs/heads/main'
           }}" >> $GITHUB_OUTPUT
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Upgrade jinja
         run: pip install -U jinja2