Documentation: Add content on Terraform

Per Sid, we need a doc on how to manage Terraform recipes with GitLab.

Let's clarify the use cases of interest, the ones we want to cover (or link out or cross-post), and the correct terminology -- are Terraform recipes more commonly called "templates"?

Existing Docs (for inspiration and potential linking or cross-posting)

• https://hackernoon.com/using-terraform-for-gitlab-review-apps-acf05920a264
• https://medium.com/@timhberry/terraform-pipelines-in-gitlab-415b9d842596
• Configuring GitLab as a 'provider' in Terraform https://www.terraform.io/docs/providers/gitlab/
• Configuring GitLab EE and CE Access in Terraform https://www.terraform.io/docs/enterprise/vcs/gitlab-eece.html

Key Use Cases (and whether/how to support via docs)

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