ci/azure-job-setup.yml: need to check out submodules for docs build

ci/azure-job-setup.yml

@@ -1,4 +1,4 @@
-# Copyright 2020 the .NET Foundation
+# Copyright 2020-2021 the .NET Foundation
 # Licensed under the MIT License
 
 # Setup steps for build and release jobs.
@@ -20,6 +20,7 @@ steps:
 - download: current
 
 - checkout: self
+  submodules: recursive
 
 - bash: |
     set -euo pipefail