Pin actions/checkout

Author: AArnott

.github/workflows/docs_validate.yml

@@ -13,7 +13,7 @@ jobs:
     name: 📚 Doc validation
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
       with:
         fetch-depth: 0 # avoid shallow clone so nbgv can do its work.
     - name: 🔗 Markup Link Checker (mlc)