Voice Agents docs URL migration - atoms → voice-agents, atoms-platform → platform

The Voice Agents product now lives under /voice-agents/ on the docs site, replacing the older /atoms/ prefix. The main tab’s URL also drops the brand doubling: /atoms/atoms-platform/* becomes /voice-agents/platform/*. All other tabs move too. /atoms/developer-guide/* becomes /voice-agents/developer-guide/*. Same for api-reference, mcp, integrations, developer-tools, and changelog.

Every old URL 301-redirects to its new equivalent. Bookmarks, external links, and search-indexed pages keep resolving. The migration matches the industry convention (Retell, ElevenLabs, Vapi, AssemblyAI all use generic function-based top-level slugs); no peer uses the brand-doubled shape our old URLs had.

API endpoints and SDK identifiers are unchanged. POST /atoms/v1/agent, AtomsClient, from smallestai.atoms import AtomsClient, @smallest/atoms - all still the same. Only docs URLs moved.