chore: Document reason why reverting back to checkout v5

Author: jcs090218

.github/workflows/docs.yml

@@ -24,6 +24,7 @@ jobs:
     - name: Build website
       run: siradoc docs build
 
+    # XXX: The checkout v6 is broken; therefore, we can't push to `gh-pages`.
     - name: Clone branch gh-pages
       uses: actions/checkout@v5
       with: