fix(docs): fix missing content from submodules

- currently the mdbook pages are missing things from the git submodules
  because they are not checked out

Signed-off-by: AtomicFS <vojtech.vesely@9elements.com>

.github/workflows/documentation-pages.yml

@@ -39,7 +39,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
-          fetch-depth: 0
+          submodules: 'recursive'
 
       - name: Restore cached mdbook
         uses: actions/cache/restore@v4