Move Terraform state doc to new location

- Move doc (https://gitlab.com/gitlab-org/gitlab/-/issues/337121)
- Move image
- Update links

doc/user/infrastructure/iac/index.md

@@ -65,7 +65,7 @@ Amazon S3 or Google Cloud Storage. Its features include:
 - Locking and unlocking state.
 - Remote Terraform plan and apply execution.
 
-Read more on setting up and [using GitLab Managed Terraform states](../terraform_state.md)
+Read more on setting up and [using GitLab Managed Terraform states](terraform_state.md).
 
 ## Terraform module registry
 

doc/user/infrastructure/index.md

@@ -27,7 +27,7 @@ The various GitLab integrations help you:
 
 Read more about the [Infrastructure as Code features](iac/index.md), including:
 
-- [The GitLab Managed Terraform State](terraform_state.md).
+- [The GitLab Managed Terraform State](iac/terraform_state.md).
 - [The Terraform MR widget](mr_integration.md).
 - [The Terraform module registry](../packages/terraform_module_registry/index.md).