docs: add stylesheet (#1706)

Author: YousefED

docs/pages/docs/ai/getting-started.mdx

@@ -59,6 +59,7 @@ import { en as aiEn } from "@blocknote/xl-ai/locales";
 import {
   createAIExtension,
 } from "@blocknote/xl-ai";
+import "@blocknote/xl-ai/style.css"; // add the AI stylesheet
 
 const editor = createBlockNoteEditor({
   dictionary: {