Update action/checkout to v6

Author: cregouby

.github/workflows/pkgdown.yaml

@@ -22,7 +22,7 @@ jobs:
       TORCH_INSTALL: 1
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up Quarto
         uses: quarto-dev/quarto-actions/setup@v2