Localize Building agents quickstart (non-EN)

Summary

Soft follow-up: _quickstart/building_agents.md shipped with English content and i18n title/description keys (#84 (closed) / MR !180 (merged)). Full non-EN body translations are still missing for locales the portal already supports (es, hi, de, fr, ru, zh, …).

Proposed scope

1. Translate building_agents body for existing portal locales (or mark clearly EN-only until done).
2. Keep links (WAX Mintlify llms, Workerbee, OpenRPC once #85 (closed) lands) locale-stable where possible.
3. Avoid regressing Jekyll/i18n CI.

Acceptance

• Non-EN locales either have translated Building agents pages or an explicit EN fallback that doesn’t 404
• Titles/descriptions already in _i18n stay consistent

Out of scope

• Translating the entire tutorial corpus