Update chat usage.

Author: Layoric

MyApp/_pages/ai-server/usage/index.md renamed to MyApp/_pages/ai-server/usage/chat.md

@@ -13,12 +13,17 @@ To make a chat request to AI Server, you can use the `/api/CreateOpenAiChat` end
 ```csharp
 var response = client.Post(new CreateOpenAiChat {
     Request = new OpenAiChat {
-        Model = "gpt-3.5-turbo",
+        Model = "gpt-4-turbo",
         Messages = new List<OpenAiMessage> {
             new OpenAiMessage { Role = "system", Content = "You are a helpful AI assistant." },
-            new OpenAiMessage { Role = "user", Content = "How do I ..." }
-        }
-    }
+            new OpenAiMessage { Role = "user", Content = "How do LLMs work?" }
+        },
+        MaxTokens = 50
+    },
+    RefId = "<unique-id>",
+    Provider = "openai",
+    ReplyTo = "https://myapp.com/reply",
+    Sync = true
 });
 ```
 
@@ -28,21 +33,16 @@ Additional optional features on the request to enhance the usage of AI Server in
 - **Provider**: Force the request to use a specific provider, overriding the selection logic.
 - **ReplyTo**: A HTTP URL to send the response to on completion of the request.
 - **Tag**: A tag to help categorize the request for easier tracking.
+- **Sync**: A boolean value to determine if the request should be processed synchronously (default is asynchronous returning job info).
+
+These additional request properties are consistent across all AI Server endpoints, providing a common interface for all AI services.
 
 ## Using the AI Server Request DTOs with other OpenAI compatible APIs
 
 One advantage of using AI Server is that it provides a common set of request DTOs in 11 different languages that are compatible with OpenAI's API. This allows you to switch between OpenAI and AI Server without changing your client code.
 This means you can switch to using typed APIs in your preferred language with your existing service providers OpenAI compatible APIs, and optionally switch to AI Server when you're ready to self-host your AI services for better value.
 
-```csharp
-// Using OpenAI API
-var request = new OpenAiChat {
-    Model = "gpt-3.5-turbo",
-    Messages = new List<OpenAiMessage> {
-        new OpenAiMessage { Role = "system", Content = "You are a helpful AI assistant." },
-        new OpenAiMessage { Role = "user", Content = "How do I ..." }
-    }
-};
-
-var response = await client.PostAsync("https://api.openai.com/v1/", request);
-```
+::include ai-server/cs/open-ai-requests-1.cs.md::
+
+This shows usage of the `OpenAiChat` request DTO directly with OpenAI's API. The same request DTO can be used with AI Server's `/api/CreateOpenAiChat` endpoint, populated the same way.
+