docs(mobile-client): add guides index page

- Create new index page for developer guides in the mobile client section
- Add CardGrid component to display available guides
- Include guides for styling/theming and localization

Author: fulleni

src/content/docs/mobile-client/guides/index.mdx

@@ -0,0 +1,19 @@
+---
+title: Developer Guides
+description: Practical guides for developers on theming, customization, and localization for the mobile client.
+---
+
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+This section provides practical, step-by-step guides for developers working on the mobile client.
+
+<CardGrid>
+  <Card title="Styling & Theming" icon="palette">
+    Learn how to customize the application's visual appearance.
+    [Read more &rarr;](/docs/mobile-client/guides/styling-and-theming/)
+  </Card>
+  <Card title="Localization" icon="translate">
+    A guide to adding new languages and translations to the mobile client.
+    [Read more &rarr;](/docs/mobile-client/guides/localization/)
+  </Card>
+</CardGrid>