chore: add documentation on Azure and Terraform

docs/deployment/azure.md

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+# Deployment into Azure
+
+[dev-benefits.calitp.org](https://dev-benefits.calitp.org) is currently deployed into a Microsoft Azure account provided by [California Department of Technology (CDT)'s Office of Enterprise Technology (OET)](https://techblog.cdt.ca.gov/2020/06/cdt-taking-the-lead-in-digital-transformation/), a.k.a. the "DevSecOps" team. More specifically, it uses [custom containers](https://docs.microsoft.com/en-us/azure/app-service/configure-custom-container) on [Azure App Service](https://docs.microsoft.com/en-us/azure/app-service/overview).
+
+The infrastructure is configured as code via [Terraform](https://www.terraform.io/), for [various reasons](https://techcommunity.microsoft.com/t5/fasttrack-for-azure/the-benefits-of-infrastructure-as-code/ba-p/2069350). We are adding existing resources to the configuration progressively. In other words, not _all_ our resources in Azure show up under [`terraform/`][terraform-dir], but we are [moving that direction](https://github.com/cal-itp/benefits/issues/618).
+
+## Making changes
+
+1. Get access to the Azure account through the DevSecOps team.
+1. Install dependencies:
+    - [Azure CLI](https://docs.microsoft.com/en-us/cli/azure/install-azure-cli)
+    - [Terraform](https://www.terraform.io/downloads)
+1. [Authenticate using the Azure CLI.](https://registry.terraform.io/providers/hashicorp/azuread/latest/docs/guides/azure_cli)
+1. Outside the [dev container](../../getting-started/), navigate to the [`terraform/`][terraform-dir] directory.
+1. [Initialize Terraform.](https://www.terraform.io/cli/commands/init)
+
+    ```sh
+    terraform init
+    ```
+
+1. Make changes to Terraform files.
+1. [Plan](https://www.terraform.io/cli/commands/plan)/[apply](https://www.terraform.io/cli/commands/apply) the changes, as necessary.
+
+    ```sh
+    terraform apply
+    ```
+
+1. [Submit the changes via pull request.](../development/commits-branches-merging/) Be sure to specify whether they've been applied, i.e. whether they're live or not.
+
+[terraform-dir]: https://github.com/cal-itp/benefits/tree/dev/terraform