Add dashboard screenshots to README

[AlexCheema]

Motivation

The README showcases exo's features and benchmarks but doesn't show what the dashboard actually looks like. Adding a screenshot helps users understand what they'll get when they run exo.

Changes

• Added dashboard screenshot to docs/imgs/dashboard-cluster-view.png: Shows the cluster topology view with 4 × 512GB M3 Ultra Mac Studio running DeepSeek v3.1 (8-bit) and Kimi-K2-Thinking (4-bit)
• Added a new "Dashboard" section to README.md below Features, displaying the screenshot with caption

Why It Works

Visual documentation helps users understand what exo offers before they install it. The screenshot demonstrates the cluster management capabilities.

Test Plan

Manual Testing

• Verified image renders correctly in GitHub markdown preview

Automated Testing

• N/A - documentation only change