Auto-publish pre-built demo images

## Summary

Set up GitHub Actions workflow to automatically publish pre-built demo Docker images to Docker Hub, reducing first-run time from 10+ minutes to instant pull.

## Proposed solution

### Why

- Current: Users build locally → 5-10 min, downloads ~4GB
- After: `docker pull selfpatch/turtlebot3-fleet-demo:latest` → 30 sec
