Fix docs CI to checkout submodules

The spec content lives in a submodule at docs/spec. Without
submodules: true the checkout is empty and Pages deploy fails.

Author: callumalpass

.github/workflows/docs.yaml

@@ -21,6 +21,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+      with:
+        submodules: true
 
     - name: Setup Node
       uses: actions/setup-node@v4