refactor: rename project to shroud

Rename project from outline-distro to shroud for better branding.

- Update module path in go.mod from outline-distro to shroud.
- Rename binary, service file, and all installation paths.
- Update all import paths across internal packages.
- Add comprehensive README.md with usage and architecture docs.
- Update install script and systemd unit file for new naming.
- Update config example with new cert cache path.