docs(api-server): add introductory content for API Server

- Create new markdown file for API Server documentation
- Add overview of API Server and its role in Flutter News App ecosystem
- Include information about Dart Frog framework and key features
- Provide links to related documentation sections

Author: fulleni

src/content/docs/api-server/index.mdx

@@ -0,0 +1,25 @@
+---
+title: API Server
+description: An overview of the high-performance Dart Frog backend that powers the Flutter News App ecosystem.
+---
+
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+The API Server is the core backend component of the Flutter News App Toolkit. Built with the high-performance [Dart Frog](https://dartfrog.vgv.dev/) framework, it provides all the necessary server-side features, including robust authentication, data management, and dynamic remote configuration.
+
+It is designed to work seamlessly with the [Mobile Client](/mobile-client/) and the [Web Dashboard](/web-dashboard/).
+
+<CardGrid>
+  <Card title="Getting Started" icon="rocket">
+    Follow the local setup guide to get the server running on your machine.
+    [Read more &rarr;](/api-server/getting-started/)
+  </Card>
+  <Card title="Deployment" icon="cloud">
+    Learn how to deploy the API server to a production environment.
+    [Read more &rarr;](/api-server/deployment/)
+  </Card>
+  <Card title="Features" icon="puzzle">
+    Explore the key features and architecture of the API server.
+    [Read more &rarr;](/api-server/features/authentication/)
+  </Card>
+</CardGrid>