Chat: describe Message Streaming demo

apps/demos/Demos/Chat/MessageStreaming/description.md

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+This demo integrates the DevExtreme Chat component with [Azure OpenAI](https://azure.microsoft.com/en-us/products/ai-services/openai-service) to stream AI-generated responses in real time. As the AI model produces output token by token, the Chat component renders the response incrementally inside a message bubble. A typing indicator appears while the response begins, and the send button transforms into a stop button so users can cancel an in-progress stream at any time.
+
+The empty state displays custom suggestion cards. Clicking a card sends the corresponding prompt directly to the AI without extra user input.
+
+<!--split-->
+
+## Streaming AI Responses
+
+The demo calls the Azure OpenAI Chat Completions API with `stream: true`. Incoming delta chunks are passed through a `createDelayedRenderer` queue that introduces a short display delay between chunks to produce a smooth typing effect. Each chunk is appended to a growing buffer, and the assistant message is updated in the data store with every render cycle via a `store().push` update operation.
+
+## Stopping a Stream
+
+An `AbortController` is created before each request and its signal is forwarded to the Azure OpenAI SDK. When the user clicks the stop button, `abortController.abort()` is called, which both cancels the HTTP request and stops the delayed renderer queue.
+
+The [sendButtonOptions](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#sendButtonOptions) property switches the button's `action` to `'custom'` and its icon to `'stopfilled'` while streaming is active, then reverts to the default send configuration once streaming ends.
+
+## Custom Empty View
+
+The Chat component specifies an [emptyViewTemplate](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#emptyViewTemplate) that replaces the default empty state with custom suggestion cards. Clicking a card constructs a message and calls `fetchAIResponse` directly, bypassing the text input.