WIP: Add services as applications explanation and example

sites/upsun/src/add-services/_index.md

@@ -11,9 +11,9 @@
 Because the services are included in your project, you can manage them through Git
 and they're backed up together with the rest of your project.
 
 Your project defines the services configuration from a top-level key called `services`, which is placed in a unified configuration file like `{{< vendor/configfile "services" >}}`.
 
 If you don't need any services (such as for a static website), you don't need to include this configuration. Read on to see how to add services.
 
 ## Add a service
 
@@ -47,7 +47,7 @@
 
 This YAML file contains a dictionary defining all of the services you want to use.
 The top-level key `services` defines an object of all of the services to be provisioned for the project.
 Below that, come custom service names ({{<variable "SERVICE_NAME" >}}; in the example, `mariadb` and `postgresql`), which you use to identify the service in step 2.
 
 You can give it any name you want with lowercase alphanumeric characters, hyphens, and underscores.
 
@@ -65,9 +65,9 @@
 
 | Name            | Type       | Required          | Description |
 | --------------- | ---------- | ----------------- | ----------- |
 | `type`          | `string`   | Yes               | One of the [available services](#available-services) in the format `type:version`. |
 | `configuration` | dictionary | For some services | Some services have additional specific configuration options that can be defined here, such as specific endpoints. See the given service page for more details. |
 | `relationships` | dictionary | For some services | Some services require a relationship to your app. The content of the dictionary has the same type as the `relationships` dictionary for [app configuration](/create-apps/app-reference/single-runtime-image.md#relationships). The `endpoint_name` for apps is always `http`. |
 
 ##### Resources (CPU, RAM, disk)
 
@@ -215,6 +215,29 @@
 <!-- To update the versions in this table, use docs/data/registry.json -->
 {{% supported-services %}}
 
+### Running other services as applications
+
+While Upsun publishes and maintains service images for the list above, you can also run any other service as an application in your project as long as they rely on a http-based interface for their respective clients.
+
+To connect through the internal network to a service running as an application from your main application in your project, add a relationship in your application configuration block. Replace `chroma` in the example below with the name of your service application.
+
+```yaml {location=".upsun/config.yaml"}
+applications:
+  myapp:
+    source:
+      root: "myapp"
+    type: "python:3.12"
+    relationships:
+      chroma: "chroma:http"
+```
+
+You can find the documented `.upsun/config.yaml` configurations and setup steps for the following services:
+
+| Name                | Type            | Runtime           |
+| ------------------- | --------------- | ----------------- |
+| [Chroma](chroma.md) | Vector Database | Python            |
+| [Qdrant](qdrant.md) | Vector Database | Composable image  |
+
 ### Service versions
 
 These services generally follow [semantic versioning conventions](https://semver.org/).