Chat: add Overview demo description (#28427)

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

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+Chat is an interactive interface that allows users to send and receive messages in real time.
+
+To get started with the DevExtreme Chat component, refer to the following step-by-step tutorial: [Getting Started with Chat](/Documentation/Guide/UI_Components/Chat/Getting_Started_with_Chat/).
+
+The demo implements basic Chat functionality: specifies initial messages, updates the conversation with new incoming and outgoing messages, manages users, and links two chats in real-time.
+<!--split-->
+
+## Messages
+
+To specify initial messages, you can either populate the [items](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#items) array (shown in this demo) or use a [dataSource](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#dataSource).
+
+Use the following API to render new messages:
+
+- If you use **items**, update the array with the new message.
+- If you use a **dataSource**, implement [load](/Documentation/ApiReference/Data_Layer/CustomStore/Configuration/#load) and [insert](/Documentation/ApiReference/Data_Layer/CustomStore/Configuration/#insert) operations.
+
+## Users
+
+To specify the chat owner, set the [user](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#user) property. Owner messages align to the right (or left in RTL mode) and do not display the name or avatar.
+
+Each message includes information about the sender ([author](/Documentation/ApiReference/UI_Components/dxChat/Types/Message/#author)): name, avatar, and alternative avatar text. If no avatar is set, the user's initials are displayed instead.
+
+## Events
+
+If a user enters a message, the Chat raises the [messageEntered](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onMessageEntered) event. Use the event handler to process the message. For example, you can display the message in the message feed and send the message to the server for storage.
+
+When users start or finish typing, the Chat raises [typingStart](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onTypingStart) and [typingEnd](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onTypingEnd) events. Use these events to manage the [typingUsers](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#typingUsers) array. The Chat uses this array to display a list of active users.

apps/demos/Demos/Chat/Overview/jQuery/description.md

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+Chat is an interactive interface that allows users to send and receive messages in real time.
+
+To get started with the DevExtreme Chat component, refer to the following step-by-step tutorial: [Getting Started with Chat](/Documentation/Guide/UI_Components/Chat/Getting_Started_with_Chat/).
+
+The demo implements basic Chat functionality: specifies initial messages, updates the conversation with new incoming and outgoing messages, manages users, and links two chats in real-time.
+<!--split-->
+
+## Messages
+
+To specify initial messages, you can either populate the [items](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#items) array (shown in this demo) or use a [dataSource](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#dataSource).
+
+Use the following API to render new messages:
+
+- If you use **items**, call the [renderMessage()](/Documentation/ApiReference/UI_Components/dxChat/Methods/#renderMessagemessage) method.
+- If you use a **dataSource**, implement [load](/Documentation/ApiReference/Data_Layer/CustomStore/Configuration/#load) and [insert](/Documentation/ApiReference/Data_Layer/CustomStore/Configuration/#insert) operations.
+
+## Users
+
+To specify the chat owner, set the [user](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#user) property. Owner messages align to the right (or left in RTL mode) and do not display the name or avatar.
+
+Each message includes information about the sender ([author](/Documentation/ApiReference/UI_Components/dxChat/Types/Message/#author)): name, avatar, and alternative avatar text. If no avatar is set, the user's initials are displayed instead.
+
+## Events
+
+If a user enters a message, the Chat raises the [messageEntered](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onMessageEntered) event. Use the event handler to process the message. For example, you can display the message in the message feed and send the message to the server for storage.
+
+When users start or finish typing, the Chat raises [typingStart](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onTypingStart) and [typingEnd](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onTypingEnd) events. Use these events to manage the [typingUsers](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#typingUsers) array. The Chat uses this array to display a list of active users.