[docs] Limit git submodule recurse to depth 1

Some projects (e.g. Fuel, Armband) embed their own git submodules,
which are picked up by Sphinx during RTD verify job, leading to a lot
of issues (e.g. duplicate labels since Armband includes Fuel as a git
submodule of its own).

Change-Id: I771f7c95334fc25c82cc8b14ee2064b043d9e321
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>