Add supervisord integration example

[ting-hong-shieh]

Closes #43.

Summary

• Add a tableflip-supervisor process proxy that stays attached to supervisord while tableflip replaces the service process.
• Forward HUP, termination, and user signals to the current PID from tableflip.Options.PIDFile.
• Exit when the managed process stops without handing off, so supervisord can restart the service.
• Include a supervisord configuration, build instructions, PID-file permission guidance, and the zero-downtime reload command.
• Explain why Supervisor's standard pidproxy exits after the first successful tableflip upgrade.

Testing

• go test ./...
• go test ./examples/supervisord -run TestProxyTracksReplacementProcess -count=25
• GOTOOLCHAIN=go1.25.5 go test -race ./...
• go vet ./...
• go build ./examples/supervisord
• Supervisor 4.3.0 end-to-end test: the service PID changed after supervisorctl signal HUP, the proxy PID remained RUNNING, and supervisorctl stop terminated both processes.