docs: add hugr.py

add namespace in  conftest.oy to allow `Dfg` use in HUGR doctests

hugr-py/src/hugr/conftest.py

@@ -0,0 +1,21 @@
+import pytest
+import hugr.hugr as hugr
+import hugr.node_port as node_port
+import hugr.dfg as dfg
+import hugr.ops as ops
+import hugr.tys as tys
+import hugr.val as val
+
+
+@pytest.fixture(autouse=True)
+def add_np(doctest_namespace):
+    doctest_namespace.update(
+        {
+            "hugr": hugr,
+            "node_port": node_port,
+            "dfg": dfg,
+            "ops": ops,
+            "tys": tys,
+            "val": val,
+        }
+    )