Add documentation for deployment slot support in Azure App Service (#290)

* Initial plan

* Add documentation for deployment slot support to Azure App Service

Co-authored-by: IEvangelist <7679720+IEvangelist@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: IEvangelist <7679720+IEvangelist@users.noreply.github.com>

Author: Copilot

src/frontend/src/content/docs/integrations/cloud/azure/azure-app-service.mdx

@@ -129,6 +129,67 @@ The preceding code:
 - Adds an application setting for `ASPNETCORE_ENVIRONMENT`.
 - Adds multiple tags for metadata and organization.
 
+## Deployment slots
+
+[Deployment slots](https://learn.microsoft.com/azure/app-service/deploy-staging-slots) let you deploy your app to a staging environment for testing before swapping it into production. The `WithDeploymentSlot` extension method configures a deployment slot for the App Service environment:
+
+```csharp title="C# — AppHost.cs"
+var builder = DistributedApplication.CreateBuilder(args);
+
+builder.AddAzureAppServiceEnvironment("appservice")
+    .WithDeploymentSlot("staging");
+
+var apiService = builder.AddProject<Projects.ApiService>("apiservice")
+    .WithHttpHealthCheck("/health")
+    .WithExternalHttpEndpoints();
+
+builder.AddProject<Projects.Web>("webfrontend")
+    .WithExternalHttpEndpoints()
+    .WithHttpHealthCheck("/health")
+    .WithReference(apiService)
+    .WaitFor(apiService);
+
+builder.Build().Run();
+```
+
+The preceding code configures all App Service websites to be deployed to a `staging` slot.
+
+### Deployment slot behavior
+
+When you deploy to a slot, the deployment behavior depends on whether the production App Service already exists:
+
+- **New App Service**: If the production App Service (production slot) doesn't exist at the time of deployment, Aspire deploys to both the production App Service and the specified deployment slot.
+- **Existing App Service**: If the production App Service already exists, Aspire deploys only to the specified deployment slot.
+
+This behavior allows you to safely deploy and test changes in a staging environment before swapping to production.
+
+### Using parameters for deployment slots
+
+You can also specify the deployment slot value as a parameter, allowing the slot name to be provided at deployment time:
+
+```csharp title="C# — AppHost.cs"
+var builder = DistributedApplication.CreateBuilder(args);
+
+var slotName = builder.AddParameter("deploymentSlot");
+
+builder.AddAzureAppServiceEnvironment("appservice")
+    .WithDeploymentSlot(slotName);
+
+// Add your projects and other resources...
+
+builder.Build().Run();
+```
+
+With this configuration, you can specify the slot name when running `aspire deploy` by providing a value for the `deploymentSlot` parameter.
+
+### Applying customizations to slots
+
+<Aside type="caution">
+Customizations applied to the production App Service are not automatically applied to deployment slots. You must explicitly apply any customizations to the slot as well.
+</Aside>
+
+When you customize App Service resources using `PublishAsAzureAppServiceWebsite` or `ConfigureInfrastructure`, those customizations apply to the production slot by default. If you need the same customizations on your deployment slot, you must apply them separately. This gives you fine-grained control over the configuration of each slot, allowing staging and production environments to have different settings when needed.
+
 ## Provisioning-generated Bicep
 
 If you're new to [Bicep](https://learn.microsoft.com/azure/azure-resource-manager/bicep/overview), it's a domain-specific language for defining Azure resources. With Aspire, you don't need to write Bicep by-hand, instead the provisioning APIs generate Bicep for you. When you publish your app, the generated Bicep is output alongside the manifest file.