[Docs] README API Documentation index omits backend/docs/AUTHENTICATION.md

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

backend/docs/AUTHENTICATION.md documents the real auth flow required to call protected endpoints, but it is orphaned - no other doc links to it, and README's API Documentation section lists only API_VERSIONING, DEPRECATION_POLICY, and SANDBOX_MODE. Contributors have no entry point to the auth model from the README.

Acceptance criteria

• Add a link to backend/docs/AUTHENTICATION.md in the README API Documentation section
• Optionally cross-link it from docs/ARCHITECTURE.md's Authentication Flow section

Files to touch

• README.md
• backend/docs/AUTHENTICATION.md

Out of scope

• Rewriting the auth implementation or AUTHENTICATION.md content