DONT MERGE: docs: add docs for cross account role

docs/infracost_cloud/cross_account_role.md

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+---
+slug: cross_account_role
+title: Cross account role
+---
+
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+Infracost supports gathering right sizing recommendations for resources in your AWS account by assuming a cross account role. This is allows us to access your AWS resources and provide you with recommendations in your code based on Trusted Advisor and Compute Optimizer data.
+
+:::info
+This feature is currently early stage, please email [[email protected]](mailto:[email protected]) for more information.
+:::
+
+For more information about cross account roles, please see the [AWS documentation](https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_common-scenarios_aws-accounts.html)
+
+## Setup of the cross account role
+
+We have provided a Terraform module to help you set up the cross account role. You can find the module [here](https://github.com/infracost/cross-account-link)
+
+### Prerequisites
+
+- You have an AWS account
+- You need your Infracost Cloud organization ID - find this in the settings of the [Infracost Dashboard](https://dashboard.infracost.io)
+
+### Steps
+
+1. Use the module to create the cross account role in your AWS account
+
+   ```hcl
+   provider "aws" {
+    region = "us-west-2"
+   }
+
+    module "infracost" {
+        source                = "github.com/infracost/cross-account-link?ref=0.1.0"
+        infracost_external_id = "INFRACOST_ORGANIZATION_ID"
+
+        providers = {
+            aws = aws
+        }
+    }
+
+    output "infracost_cross_account_role_arn" {
+        value = module.infracost.role_arn
+    }
+   ```
+
+2. Run `terraform init` and `terraform apply` to create the cross account role
+3. Share the `infracost_cross_account_role_arn` output with the Infracost team by email email [[email protected]](mailto:[email protected])