docs: revamp README with feature lists, detailed examples, and a visual showcase

[oca-agent]

This PR completely revamps the README.md with detailed feature lists, an updated complete code example, and a high-quality visual showcase of the library's features.

Improvements Made:

1. Feature Lists & Architecture Details: Clearly explained the library's capabilities (such as EGBI segmentation, global/regional visual saliency, Gaussian blur, Lab* conversion, and platform-specific packages) and structured the repository's layout.
2. Updated Detailed Example: Rewrote the main code example to show a complete, clean end-to-end flow utilizing modern SixLabors ImageSharp 3.x constructs and platform extensions.
3. Visual Showcase:
   • Dynamically generated a highly-detailed studio shot of a bright red tulip with green leaves.
   • Processed the tulip through the entire Leiter suite (Gaussian Blur -> EGBI Segmentation with Average Color Assigner -> Global Contrast Saliency -> Regional Contrast Saliency).
   • Placed the original, segmented, and both visual saliency outputs in the newly created examples/ directory and embedded them as a side-by-side comparison table directly in the README.