explain static assets proxying

Author: Erwinvandervalk

src/content/docs/bff/fundamentals/multi-frontend/index.mdx

@@ -155,14 +155,59 @@ var frontend = new BffFrontend(BffFrontendName.Parse("frontend1"))
 
 See the topic on [Token Management](/bff/fundamentals/tokens.md) for more information about the various token management options. 
 
-## Index HTML Proxying
+## Handling SPA static assets
+
+The BFF can be configured to handle the static file assets that are typically used when developing SPA based apps. 
+
+### Proxying the Index.HTML only
 
 When deploying a multi-frontend BFF, it makes most sense to have the frontend's configured with an `index.html` file that is retrieved from a Content Delivery Network (CDN). 
 
+This can be done in various ways. For example, if you use Vite, you can publish static assets with a 'base' configured. This will make sure that any static asset,
+(such as images, scripts, etc) are retrieved directly from the CDN. This gives the best performance. 
+
 ```csharp
 var frontend = new BffFrontend(BffFrontendName.Parse("frontend1"))
    .WithCdnIndexHtml(new Uri("https://my_cdn/some_app/index.html"))
 ```
 
 When you do this, the BFF automatically wires up a catch-all route that serves the `index.html` for that specific frontend. 
 See [Serve the index page from the BFF host](/bff/architecture/ui-hosting.md#serve-the-index-page-from-the-bff-host) for more information. 
+
+### Proxying all static assets 
+
+When developing a SPA based application, it's very common to use a development webserver such as Vite. While vite can publish static assets with a 'base',
+this doesn't work well during development.
+
+The best development experience can be achieved by configuring the BFF to proxy all static assets from the development server:
+
+
+```csharp
+var frontend = new BffFrontend(BffFrontendName.Parse("frontend1"))
+   .WithProxiedStaticAssets(new Uri("https://localhost:3000")); // https://localhost:3000 would be the URL of your development web server.
+```
+
+While this can also be done in production, it will proxy all static assets through the BFF. This will increase the bandwidth consumed by the BFF 
+and reduce the overall performance of your application. 
+
+
+### Proxying assets based on environment
+
+If you're using a local development server during development, but a CDN in production, you can also configure this as follows:
+
+``` csharp
+
+// In this example, the environment name from the application builder is used to determine
+// if we're running in production or not. 
+var runningInProduction = () => builder.Environment.EnvironmentName == Environments.Production;
+
+// Then, when configuring the frontend, you can switch when the static assets will be proxied. 
+new BffFrontend(BffFrontendName.Parse("default-frontend"))
+    .WithBffStaticAssets(new Uri("https://localhost:5010/static"), useCdnWhen: runningInProduction);
+
+```
+
+:::note
+This function is evaluated immediately when calling the method `.WithBffStaticAssets` is called. If you call this method during startup,
+then the condition is only evaluated at startup time. It's not evaluated at runtime for every request. 
+:::