chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docs.yml

@@ -25,7 +25,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install mdBook
         run: |