[Docs] Document systemd service usage

[marctjones]

Summary

Systemd service files exist but usage is not documented.

Files Exist

• systemd/ebpf-assistd.service - System-wide daemon
• systemd/ebpf-assistd.user.service - User service
• systemd/ebpf-assistd@.service - Per-user template

Need to Document

1. How to install the service files
2. How to enable/start the daemon
3. Difference between system vs user service
4. How capabilities are handled with systemd
5. Checking logs with journalctl
6. Troubleshooting service issues

Example Section

## Running as a Systemd Service

### User Service (recommended)
# Install
mkdir -p ~/.config/systemd/user
cp systemd/ebpf-assistd.user.service ~/.config/systemd/user/
systemctl --user daemon-reload
systemctl --user enable --now ebpf-assistd

# Check status
systemctl --user status ebpf-assistd
journalctl --user -u ebpf-assistd -f