docs(mobile-client): add search feature documentation

- Create new documentation page for the search feature
- Explain how users can find headlines, topics, and sources
- Mention the unified interface for searching across the entire application
- Set up structure for future content on core components

Author: fulleni

src/content/docs/mobile-client/features/search.mdx

@@ -0,0 +1,10 @@
+---
+title: Search
+description: Learn how the search feature allows users to find headlines, topics, and sources.
+---
+import { Card, CardGrid } from '@astrojs/starlight/components';
+
+The Search feature provides a powerful tool for users to find content across the entire application. Users can search for specific headlines, discover new topics, or look up their favorite news sources, all from a single, unified interface.
+
+## Core Components
+