docs: add app overview

docs/app/overview.md

@@ -3,3 +3,65 @@ sidebar_position: 10
 ---
 
 # Overview
+
+The VOL service is distributed across three PHP applications built on the Laminas MVC framework:
+
+-   **API**: The backend service.
+-   **Selfserve**: The public-facing application.
+-   **Internal**: The internal-facing application.
+
+These applications are supported by several internal libraries.
+
+## Internal Dependencies
+
+| Library                           | Description                                                            | Link                                                                   |
+| --------------------------------- | ---------------------------------------------------------------------- | ---------------------------------------------------------------------- |
+| `authentication-cognito`          | PHP adapter for AWS Cognito authentication                             | [GitHub](https://github.com/dvsa/authentication-cognito)               |
+| `authentication-ldap`             | PHP adapter for LDAP authentication                                    | [GitHub](https://github.com/dvsa/authentication-ldap)                  |
+| `laminas-config-cloud-parameters` | Replaces Laminas configuration placeholders with cloud provider values | [GitHub](https://github.com/dvsa/dvsa-laminas-config-cloud-parameters) |
+| `olcs-auth`                       | Authentication management                                              | [GitHub](https://github.com/dvsa/olcs-auth)                            |
+| `olcs-common`                     | Shared code for frontend applications                                  | [GitHub](https://github.com/dvsa/olcs-common)                          |
+| `olcs-logging`                    | Logging utilities                                                      | [GitHub](https://github.com/dvsa/olcs-logging)                         |
+| `olcs-transfer`                   | Data Transfer Objects (DTOs)                                           | [GitHub](https://github.com/dvsa/olcs-transfer)                        |
+| `olcs-utils`                      | Shared utility functions                                               | [GitHub](https://github.com/dvsa/olcs-utils)                           |
+| `olcs-xmltools`                   | XML processing tools                                                   | [GitHub](https://github.com/dvsa/olcs-xmltools)                        |
+| `php-govuk-account`               | PHP adapter for GOV.UK One Login                                       | [GitHub](https://github.com/dvsa/php-govuk-account)                    |
+
+### Dependency tree
+
+#### API
+
+```mermaid
+flowchart LR
+    api["API"]
+
+    api --> `olcs-logging`
+    api --> `olcs-utils`
+    api --> `olcs-xmltools`
+    api --> `olcs-transfer`
+    api --> `php-govuk-account`
+    api --> `authentication-cognito`
+    api --> `authentication-ldap`
+    api --> `laminas-config-cloud-parameters`
+```
+
+#### Frontend
+
+```mermaid
+flowchart LR
+    frontend["Internal/Selfserve"]
+
+    frontend --> `olcs-common`
+    frontend --> `olcs-logging`
+    frontend --> `olcs-auth`
+    frontend --> `olcs-transfer`
+    frontend --> `olcs-utils`
+    frontend --> `laminas-config-cloud-parameters`
+
+    `olcs-common` --> `olcs-logging`
+    `olcs-common` --> `olcs-utils`
+    `olcs-common` --> `olcs-transfer`
+
+    `olcs-auth` --> `olcs-common`
+    `olcs-auth` --> `olcs-transfer`
+```