docs: add volume-reset troubleshooting and fix emulator setup hints

[jasonchen922]

What

• docs/quickstart.md: add a Troubleshooting entry — when a service stays unhealthy after an upgrade (e.g. container sico-backend is unhealthy), reset stale local Docker volumes with make compose-down-volumes, then make compose-up.
• sandbox/emulator/setup/setup.sh: replace misleading ./setup.sh <cmd> runtime hints with the documented make emulator-* targets (bootstrap/start/stop/restart/logs/setup), so guidance is correct from the repo root where they are actually invoked.

Why

• make emulator-setup runs the script from the repo root, but the script printed hints like ./setup.sh bootstrap that do not resolve there — confusing users into thinking the script is missing.
• The "backend unhealthy" failure after pulling new code (stale MySQL volume) had no documented remedy.

Notes

• Native-CLI usage/help block and script-only flags (--skip-mumu, help) intentionally kept as ./setup.sh (no make equivalent).
• No new files; existing license headers untouched. bash -n on the script passes.