Improve upgrade path docs for existing colonies

Goal

Improve the upgrade path documentation for existing colonies.

Why

The current Quick Start is optimized for greenfield installs. Existing users also need a short, safe upgrade checklist.

Scope

• Extend QUICKSTART.md
• Clarify --upgrade, backups, and post-upgrade validation
• Keep English and Chinese entry docs aligned

Done when

• Upgrading feels as straightforward as first-time installation