docs: add backend onboarding and blue-green setup guide

[cuph7022]

Summary

• add a backend developer onboarding guide covering setup, checks, CI expectations, and blue/green local scripts
• link the guide from README.md near the backend docs section

Closes #345

Validation

• Reviewed package scripts, README, and CI workflow via GitHub API.
• Not run locally: docs-only change; no dependencies were downloaded or executed on the contributor machine.

[mikewheeleer]

this onboarding + blue-green guide is genuinely useful, thank you. lgtm