docs(architecture): add architecture section with customization focus

- New documentation section on architecture and customization
- Explains multi-repository philosophy and practical guides for customization
- Includes step-by-step instructions for taking ownership of shared UI package
- Covers customizing applications across mobile and web platforms

Author: fulleni

src/content/docs/architecture/index.mdx

@@ -0,0 +1,24 @@
+---
+title: 'Architecture & Customization'
+description: Learn how the toolkit is structured and how to take full ownership of the shared packages to customize your applications.
+---
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+Welcome to the Architecture & Customization section. This is the most important part of the documentation for developers who want to go beyond the default setup and truly make the toolkit their own.
+
+Here, you will learn about the multi-repository philosophy that underpins the toolkit and, most importantly, follow a step-by-step, practical guide to customizing the shared UI package and seeing your changes reflected across both the mobile and web applications.
+
+<CardGrid>
+	<Card title="1. Understand the Toolkit Architecture" icon="file-tree">
+		Start here to learn about the multi-repository structure and why it's designed this way.
+		[Read more &rarr;](/docs/architecture/01-toolkit-architecture/)
+	</Card>
+	<Card title="2. Take Ownership of Packages" icon="key">
+		Follow this critical guide to learn how to host the shared packages in your own private repositories.
+		[Read more &rarr;](/docs/architecture/02-taking-ownership-of-packages/)
+	</Card>
+	<Card title="3. Practical Guide: Customize the UI" icon="palette">
+		Put theory into practice with a hands-on tutorial for customizing the UI theme.
+		[Read more &rarr;](/docs/architecture/03-guide-customizing-the-ui/)
+	</Card>
+</CardGrid>