Add dashboard screenshots to README (#1185)

## 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

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -27,6 +27,15 @@ exo connects all your devices into an AI cluster. Not only does exo enable runni
 - **Tensor Parallelism**: exo supports sharding models, for up to 1.8x speedup on 2 devices and 3.2x speedup on 4 devices.
 - **MLX Support**: exo uses [MLX](https://github.com/ml-explore/mlx) as an inference backend and [MLX distributed](https://ml-explore.github.io/mlx/build/html/usage/distributed.html) for distributed communication.
 
+## Dashboard
+
+exo includes a built-in dashboard for managing your cluster and chatting with models.
+
+<p align="center">
+  <img src="docs/imgs/dashboard-cluster-view.png" alt="exo dashboard - cluster view showing 4 x M3 Ultra Mac Studio with DeepSeek v3.1 and Kimi-K2-Thinking loaded" width="80%" />
+</p>
+<p align="center"><em>4 × 512GB M3 Ultra Mac Studio running DeepSeek v3.1 (8-bit) and Kimi-K2-Thinking (4-bit)</em></p>
+
 ## Benchmarks
 
 <details>