docs: add platform-backend blob storage

Author: langchain-infra

docs/self_hosting/configuration/blob_storage.mdx

@@ -55,7 +55,7 @@ Once you have the correct policy, there are two ways to authenticate with Amazon
 
 1. [**IAM Role for Service Account**](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html): You can create an IAM role for your LangSmith instance and attach the policy to that role. You can then provide the role to LangSmith. This is the recommended way to authenticate with Amazon S3 in production.
    1. You will need to create an IAM role with the policy attached.
-   1. You will need to allow LangSmith service accounts to assume the role. The `langsmith-queue` and `langsmith-backend` service accounts will need to be able to assume the role.
+   1. You will need to allow LangSmith service accounts to assume the role. The `langsmith-queue`, `langsmith-backend`, and `langsmith-platform-backend` service accounts will need to be able to assume the role.
       :::warning Service Account Names
       The service account names will be different if you are using a custom release name. You can find the service account names by running `kubectl get serviceaccounts` in your cluster.
       :::