Update checkout action to v4

checkout@v2 is deprecated

.github/workflows/docs.yml

@@ -20,7 +20,7 @@ jobs:
   docs:
     runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Use Node.js
         uses: actions/setup-node@v1
         with: { node-version: '10' }