[CI][Docs] Preserve block order during doctest (#36146)

Signed-off-by: Balaji Veeramani <balaji@anyscale.com>

Ray Data is non-deterministic. As a result, the 📖 Doctest (CPU) CI sometimes fails like this:
To decrease flakiness, this PR enables the preserve block order feature.

bazel/conftest.py

@@ -7,3 +7,9 @@
 def shutdown_ray():
     ray.shutdown()
     yield
+
+
+@pytest.fixture(autouse=True)
+def preserve_block_order():
+    ray.data.context.DataContext.get_current().execution_options.preserve_order = True
+    yield

doc/source/data/glossary.rst

@@ -17,9 +17,6 @@ Ray Data Glossary
         .. doctest::
 
             >>> import ray
-            >>> # Dataset is executed by streaming executor by default, which doesn't
-            >>> # preserve the order, so we explicitly set it here.
-            >>> ray.data.context.DataContext.get_current().execution_options.preserve_order = True
             >>> dataset = ray.data.range(10)
             >>> next(iter(dataset.iter_batches(batch_format="numpy", batch_size=5)))
             {'id': array([0, 1, 2, 3, 4])}