Add extender docs

.github/config/en-custom.txt

@@ -1077,4 +1077,6 @@ reimagines
 TechCrunch
 ZDNET
 Newstrail
+postgresql
+natively
 NuGet

docs/content/guides/author-apps/custom/_index.md

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+---
+type: docs
+title: "Custom resources"
+linkTitle: "Custom resources"
+description: "Model and deploy your own resource types"
+weight: 550
+---

docs/content/guides/author-apps/custom/howto-extenders/index.md

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+---
+type: docs
+title: "How-To: Model any resource with an extender"
+linkTitle: "Extenders"
+description: "Learn how to use extenders in an application"
+weight: 300
+---
+
+This guide will walk you through how to use an [extender]({{< ref "/guides/author-apps/custom/overview#extenders" >}}) in an application to model resources beyond those currently built into Radius. These might be abstractions or resources unique to your organization, or resources that don't yet have a native Radius type.
+
+## Prerequisites
+
+- [rad CLI]({{< ref "installation#step-1-install-the-rad-cli" >}})
+- [Radius Bicep VSCode extension]({{< ref "installation#step-2-install-the-vs-code-extension" >}})
+- [Radius environment]({{< ref "installation#step-3-initialize-radius" >}})
+
+## Step 1: Register an extender Recipe
+
+In this guide you will leverage a [Recipe]({{< ref "/guides/recipes/overview" >}}) to deploy backing infrastructure for your resource. Begin by registering the 'extender-postgresql' Recipe in your environment:
+
+```bash
+rad recipe register postgresql --resource-type "Applications.Core/extenders" --template-kind bicep --template-path "ghcr.io/radius-project/recipes/local-dev/postgresql:latest"
+```
+
+## Step 2: Define an extender
+
+Open a new file named `app.bicep` and define an extender:
+
+{{< rad file="snippets/app.bicep" embed=true >}}
+
+## Step 3: Add a container
+
+Add a container to your `app.bicep` file, accessing the extender's properties and secrets:
+
+{{< rad file="snippets/app-container.bicep" embed=true marker="//CONTAINER" >}}
+
+## Step 4: Deploy the app
+
+Deploy and [run]({{< ref rad_run >}}) the app using the following command:
+
+```bash
+rad run app.bicep -a demo
+```
+
+You should see your application deployed:
+
+```
+Building .\app.bicep...
+Deploying template '.\app.bicep' for application 'demo' and environment 'default' from workspace 'default'...
+
+Deployment In Progress...
+
+
+Deployment Complete
+
+Resources:
+    demo            Applications.Core/containers
+    extender        Applications.Core/extenders
+
+Starting log stream...
+
+demo-bb9df8798-b68rc › demo
+demo-bb9df8798-b68rc demo Using in-memory store: no connection string found
+demo-bb9df8798-b68rc demo Server is running at http://localhost:3000
+demo-bb9df8798-b68rc demo [port-forward] connected from localhost:3000 -> ::3000
+```
+
+## Step 5: Test the app
+
+Visit [https://localhost:3000](https://localhost:3000) to see your app running. You should see the environment variables and secrets you referenced in your container.
+
+## Cleanup
+
+To clean up the resources created in this guide, run:
+
+```bash
+rad app delete demo -y
+```

docs/content/guides/author-apps/custom/howto-extenders/snippets/app-container.bicep

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+import radius as rad
+
+@description('The ID of your Radius environment. Set automatically by the rad CLI.')
+param environment string
+
+@description('The ID of your Radius application. Set automatically by the rad CLI.')
+param application string
+
+resource extender 'Applications.Core/extenders@2023-10-01-preview' = {
+  name: 'postgresql'
+  properties: {
+    environment: environment
+    application: application
+    recipe: {
+      name: 'postgresql'
+    }
+  }
+}
+
+//CONTAINER
+resource demo 'Applications.Core/containers@2023-10-01-preview' = {
+  name: 'demo'
+  properties: {
+    application: application
+    container: {
+      image: 'ghcr.io/radius-project/samples/demo:latest'
+      env: {
+        POSTGRESQL_HOST: extender.properties.host
+        POSTGRESQL_PORT: extender.properties.port
+        POSTGRESQL_USERNAME: extender.properties.username
+        POSTGRESQL_PASSWORD: extender.secrets('password')
+      }
+      ports: {
+        web: {
+          containerPort: 3000
+        }
+      }
+    }
+    connections: {
+      postgresql: {
+        source: extender.id
+      }
+    }
+  }
+}
+//CONTAINER

docs/content/guides/author-apps/custom/howto-extenders/snippets/app.bicep

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+import radius as rad
+
+@description('The ID of your Radius environment. Set automatically by the rad CLI.')
+param environment string
+
+@description('The ID of your Radius application. Set automatically by the rad CLI.')
+param application string
+
+resource extender 'Applications.Core/extenders@2023-10-01-preview' = {
+  name: 'postgresql'
+  properties: {
+    environment: environment
+    application: application
+    recipe: {
+      name: 'postgresql'
+    }
+  }
+}

docs/content/guides/author-apps/custom/overview/index.md

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+---
+type: docs
+title: "Overview: Custom resource types"
+linkTitle: "Overview"
+description: "Learn how to model and deploy your own resource types in Radius"
+weight: 100
+---
+
+When you have a need for types beyond those currently built into Radius, you can model and deploy your own resource types. Currently, Radius supports an untyped "extender" resource which allows you to pass in any property or secret.
+
+## Extenders
+
+Extenders are a special type of resource that allows you to pass in any property or secret. This is useful for modeling resources that aren't natively supported by Radius.
+
+[Recipes]({{< ref "/guides/recipes/overview" >}}) can be used with extenders to manage the backing infrastructure.
+
+Refer to the [How-To: Extenders guide]({{< ref howto-extenders >}}) for more information on how to use an extender.