Simplify README and add CLI demo GIF

[Benziza]

Summary

• Replace the long README with a short introduction, quick start, and in-app example.
• Add a terminal GIF showing actual CLI output for an overridden configuration key.

Verification

• GIF inspected at 960×540 and 27 frames.
• dotnet pack src/WhyConfig.Cli -c Release succeeds with the updated package README.