Added external link for Terraform

The link points to the GitLab blog.

doc/README.md

@@ -315,6 +315,7 @@ The following documentation relates to the DevOps **Configure** stage:
 | [Protected variables](ci/variables/README.md#protected-environment-variables)                                                      | Restrict variables to protected branches and tags.                        |
 | [Serverless](user/project/clusters/serverless/index.md)                                                                            | Run serverless workloads on Kubernetes.                                   |
 | [Slack slash commands](user/project/integrations/slack_slash_commands.md)                                                          | Enable and use slash commands from within Slack.                          |
+| [Manage your infrastructure with Terraform](user/infrastructure/index.md)                                                  | Manage your infrastructure as you run your CI/CD pipeline.                |
 
 <div align="right">
   <a type="button" class="btn btn-default" href="#overview">

doc/user/infrastructure/index.md

+# Infrastructure as Code
+
+GitLab can be used to manage infrastructure as code. The following are some examples:
+
+- [A generic tutorial for Terraform with GitLab](https://medium.com/@timhberry/terraform-pipelines-in-gitlab-415b9d842596).
+- [Terraform at GitLab](https://about.gitlab.com/blog/2019/11/12/gitops-part-2/).

doc/user/project/operations/index.md

@@ -6,6 +6,7 @@ your applications:
 - Collect [Prometheus metrics](../integrations/prometheus_library/index.md).
 - Deploy to different [environments](../../../ci/environments.md).
 - Connect your project to a [Kubernetes cluster](../clusters/index.md).
+- Manage your infrastructure with [Infrastructure as Code](../../infrastructure/index.md) approaches.
 - Discover and view errors generated by your applications with [Error Tracking](error_tracking.md).
 - Create, toggle, and remove [Feature Flags](feature_flags.md). **(PREMIUM)**
 - [Trace](tracing.md) the performance and health of a deployed application. **(ULTIMATE)**