Add Professional Thumbnail Banner to README for Social Media Sharing

[himanshu231204]

Problem

The README lacks a professional visual banner/thumbnail at the top that can be shared on social media platforms (Twitter/X, LinkedIn, Reddit, Hacker News) to promote the repository. A compelling banner increases visibility and helps users immediately understand the tool's value proposition.

Proposed Solution

Add a professionally designed banner image at the top of README.md (before the title or replacing the current title section) that:

• Visually represents the tool's purpose (RAG evaluation, AI agents)
• Includes the project name "OpenAgent Eval" prominently
• Contains a tagline or key value proposition
• Uses consistent branding colors
• Is shareable on social media platforms

Specifications

• Recommended dimensions: 1280x640px (Twitter/LinkedIn optimal) or 1200x630px (Open Graph standard)
• Format: PNG or SVG (for crisp rendering)
• Placement: Top of README.md, above the badges section
• Fallback: Alt text for accessibility

Design Suggestions

• Include visual elements representing: evaluation metrics, RAG pipelines, terminal/CLI, reports
• Color scheme: Professional, tech-focused (blues, greens, or purples)
• Consider including: Logo, tool name, tagline ("pytest-level simplicity for AI evaluation")
• Make it visually distinct from generic AI/ML tool banners

Example Placement in README

<div align="center">

![OpenAgent Eval Banner](./docs/assets/banner.png)

# OpenAgent Eval
...

Acceptance Criteria

• Banner image created and added to /docs/assets/ or similar location
• Banner added to top of README.md
• Image is optimized for web (< 500KB)
• Alt text included for accessibility
• Banner looks good on both light and dark backgrounds (or has appropriate styling)

Additional Notes

• Could use tools like Figma, Canva, or SVG code
• Consider creating multiple sizes for different platforms
• May want to create a social-preview folder with platform-specific versions

[himanshu231204]

@Nitjsefnie take a look on issue#236