Add banner image to docs homepage

- Added Phala Docs banner (phala-docs-og.png) to the default page
- Banner appears at the top of the documentation homepage
- Also set as OG image for social media previews
- Rounded corners styling for better visual presentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Marvin-Cypher

index.mdx

@@ -3,8 +3,15 @@ title: Overview
 description: Official documentation for Phala Cloud - deploy AI applications with hardware-level privacy, TEE security, and verifiable computation. Get started with confidential computing today.
 cover: images/phala-docs (1).jpg
 icon: "book-open"
+image: /images/phala-docs-og.png
 ---
 
+<img
+  className="block my-0 pointer-events-none rounded-xl"
+  src="/images/phala-docs-og.png"
+  alt="Phala Documentation - Confidential AI and TEE Computing Platform"
+/>
+
 Welcome to the official documentation for **Phala** - the secure cloud platform that enables AI developers to deploy applications with hardware-level privacy and verifiable computation.
 
 ## What is Phala Cloud