chore: consider nullable-type-sibling rule warn-level

Author: barjin

redocly.yaml

@@ -10,6 +10,7 @@ rules:
     # TODO we have quite a lot of errors in there, once we fix them, this should be flipped to `error` too
     struct: warn
     no-unused-components: error
+    nullable-type-sibling: warn
 
 openapi:
     htmlTemplate: openapi/docs/index.html