Rename Docker image to fivem, simplify README, automate version badge updates

[Copilot]

The Docker image created by the automated builds was named docker_fivem but should be simply fivem. The README contained too much detail and the FiveM version badge required manual updates.

Changes

Workflow Updates

• Changed image name from ghcr.io/${{ github.repository }} to ghcr.io/${{ github.repository_owner }}/fivem
• Added automated README badge update step that extracts the recommended FiveM version and commits the updated badge after successful builds
• Badge update only runs on successful builds with proper validation and error handling

README Simplification

• Removed non-essential sections: What's Included, WebSocket info, Build Arguments, detailed Getting Started, Version History, Manual Workflow Trigger, detailed Version Pinning, Upgrading steps, Contributing
• Kept core sections: Quick Start, Environment Variables, Volume Mounts, Automatic Builds & Versions
• Updated all image references from docker_fivem to fivem
• Updated Docker badge from docker-docker__fivem to docker-fivem

The FiveM version badge will now automatically update every 2 weeks when the scheduled build runs:

- name: 📝 Update README Badge
  run: |
    ARTIFACT=$(echo "$MATRIX" | jq -r '.[0].artifact')
    sed -i "s|badge/fivem-[0-9]*-green|badge/fivem-$ARTIFACT-green|g" README.md
    git commit -m "chore: update FiveM version badge to $ARTIFACT"
    git push

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