Optimize installation pages SEO meta titles and descriptions

✨ SEO Improvements:
- Mac: 'Run AI models locally on your Mac - Jan'
- Linux: 'Run AI models locally on Linux - Jan'
- Windows: 'Run AI models locally on Windows - Jan'

🎯 Meta descriptions now include:
- Target keywords (local AI, LLM, offline, ChatGPT-like)
- Platform-specific details (Apple Silicon, Ubuntu/Debian, Windows 10/11)
- Key benefits (GPU acceleration, privacy, no internet required)

📍 Sidebar navigation titles unchanged - only SEO meta data optimized

docs/src/pages/docs/desktop/install/linux.mdx

@@ -1,6 +1,6 @@
 ---
-title: Linux
+title: Run AI models locally on Linux - Jan
-description: Get started quickly with Jan, an AI chat application that runs 100% offline on your desktop & mobile (*coming soon*).
+description: Install Jan to run large language models locally on Linux. Offline AI with GPU acceleration support for Ubuntu, Debian, and other distributions.
 keywords:
   [
     Jan,

docs/src/pages/docs/desktop/install/windows.mdx

@@ -1,6 +1,6 @@
 ---
-title: Windows
+title: Run AI models locally on Windows - Jan
-description: Run AI models locally on your Windows machine with Jan. Quick setup guide for local inference and chat.
+description: Install Jan to run large language models locally on Windows PC. Offline ChatGPT-like AI with GPU acceleration support for Windows 10 and 11.
 keywords:
   [
     Jan,