Chat: Add Suggestions Demo Desc.

Author: arman-boyakhchyan

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

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+DevExtreme [Chat](/Documentation/Guide/UI_Components/Chat/Overview/) can display suggestion buttons above the message input field. This demo implements [suggestions](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#suggestions) to define predefined prompts for an AI assistant chat.
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+<!--split-->
+This demo allows you to configure suggestion buttons as follows:
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+- Send predefined prompts immediately or fill the Chat [input field](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#inputFieldText) when a suggestion is clicked
+- Toggle single-use suggestions (buttons disappear on click)