docs: add external reference to cfn crd

Author: heitorlessa

mkdocs.yml

@@ -27,6 +27,7 @@ nav:
       - utilities/idempotency.md
       - utilities/feature_flags.md
       - utilities/jmespath_functions.md
+      - CloudFormation Custom Resources: https://github.com/aws-cloudformation/custom-resource-helper" target="_blank
 
 theme:
   name: material