[docs] document private repo second-computer bootstrap

[FlowmemoryAI]

Summary

• replace unauthenticated raw GitHub installer instructions with the private-repo bootstrap path
• document installing Git + GitHub CLI, running gh auth login, cloning, and then running INSTALL_FLOWCHAIN_WINDOWS.ps1
• add troubleshooting for raw URL 404, missing gh, and auth prompts

Validation

• powershell -NoProfile -ExecutionPolicy Bypass -File .\INSTALL_FLOWCHAIN_WINDOWS.ps1 -CheckOnly
• git diff --check