Mark our custom sphinx extension as parallel safe (#3935)

diff --git a/docs/cryptography-docs.py b/docs/cryptography-docs.py
index 56baf12..238dd69 100644
--- a/docs/cryptography-docs.py
+++ b/docs/cryptography-docs.py
@@ -59,3 +59,7 @@
         latex=(latex_visit_hazmat_node, depart_hazmat_node),
     )
     app.add_directive("hazmat", HazmatDirective)
+
+    return {
+        "parallel_read_safe": True,
+    }