CI: Include submodules in repo checkout (Docs deployment) (#6557)

* Enable submodule checkout in documentation build

* Change submodule checkout to recursive

.github/workflows/documentation-build.yml

@@ -19,6 +19,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
+        with:
+          submodules: recursive
 
       - name: Install Doxygen and Graphviz
         run: sudo apt-get install -y doxygen graphviz