docs: add CI/CD documentation (#37)

docs/app/_category_.json

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 {
   "label": "App",
-  "position": 20,
+  "position": 30,
   "collapsed": false
 }

docs/ci-cd.md

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+---
+sidebar_position: 50
+---
+
+# CI/CD
+
+At a high level, the VOL application's CI/CD pipeline can be visualised as follows:
+
+![CI/CD](./assets/ci-cd.png)
+
+## Continuous Integration (CI)
+
+:::note
+
+Only parts of the application that have changed are updated during continuous integration.
+
+:::
+
+The CI workflow is triggered by a `pull_request` to the default branch (`main`). The workflow is responsible for building, testing the application & infrastructure, and running Terraform plans on the infrastructure.
+
+**Workflow**: [.github/workflows/ci.yaml](https://github.com/dvsa/vol-app/blob/main/.github/workflows/ci.yaml).
+
+Various tools run on CI to ensure the quality of the codebase:
+
+### ![](./assets/languages/php.svg) PHP
+
+#### Testing
+
+-   [PHPUnit](https://github.com/sebastianbergmann/phpunit)
+
+#### Linting
+
+-   [PHPStan](https://github.com/phpstan/phpstan)
+-   [Psalm](https://github.com/vimeo/psalm)
+-   [PHP_CodeSniffer](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer)
+
+#### Security
+
+-   [Snyk](https://snyk.io/)
+
+### ![](./assets/languages/docker.svg) Docker
+
+#### Linting
+
+-   [Hadolint](https://github.com/hadolint/hadolint)
+
+#### Testing
+
+-   Docker build (`docker build`)
+
+#### Security
+
+-   [Trivy](https://github.com/aquasecurity/trivy)
+-   [Checkov](https://github.com/bridgecrewio/checkov)
+
+### ![](./assets/languages/terraform.svg) Terraform
+
+#### Linting
+
+-   [TFLint](https://github.com/terraform-linters/tflint)
+-   Terraform format (`terraform fmt`)
+
+#### Testing
+
+-   Terraform validate (`terraform validate`)
+-   Terraform plan (`terraform plan`)
+
+#### Security
+
+-   [Snyk](https://snyk.io/)
+-   [Trivy](https://github.com/aquasecurity/trivy)
+-   [Checkov](https://github.com/bridgecrewio/checkov)
+
+## Continuous Deployment (CD)
+
+The CD workflow is triggered by a successful merge to the default branch (`main`). The workflow is responsible for building and deploying the application through to the production environment.
+
+**Workflow**: [.github/workflows/cd.yaml](https://github.com/dvsa/vol-app/blob/main/.github/workflows/cd.yaml).
+
+![CD workflow](./assets/cd.png)

docs/infrastructure/_category_.json

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 {
   "label": "Infra",
-  "position": 30,
+  "position": 40,
   "collapsed": false
 }

docs/infrastructure/docker/_category_.json

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+{
+  "label": "Docker",
+  "position": 10,
+  "collapsed": false
+}

docs/infrastructure/terraform/_category_.json

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+{
+  "label": "Terraform",
+  "position": 20,
+  "collapsed": false
+}

docs/infrastructure/terraform/overview.md

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+# Overview