docs: add TrustedRouter provider setup

[jperla]

Summary

• Add a TrustedRouter provider setup page.
• Add TrustedRouter to the model providers index.
• Document the existing OpenRouter provider + custom base URL flow with https://api.trustedrouter.com/v1.

Context

This follows the maintainer redirect from Zoo Code core to Zoo Code Docs: Zoo-Code-Org/Zoo-Code#594

Validation

• pnpm install --frozen-lockfile
• pnpm run build

[jperla]

Friendly follow-up on this docs PR. This was redirected here from the main Zoo Code repo and keeps the TrustedRouter setup docs in the docs repo. Happy to adjust wording or placement if you prefer another format.