ci: Revert checkdoc to v5

Author: jcs090218

.github/workflows/docs.yml

@@ -25,7 +25,7 @@ jobs:
       run: siradoc docs build
 
     - name: Clone branch gh-pages
-      uses: actions/checkout@v6
+      uses: actions/checkout@v5
       with:
         path: 'gh-pages'
         ref: gh-pages