Add versioned docs for v2.1 (#1903) Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

docs(embeddings): replace deprecated Google text-embedding-004 with gemini-embedding-2 Google shut down text-embedding-004 on 2026-01-14. Update all Google AI embedding examples to default to google:gemini-embedding-2 (the latest GA model) and the legacy example to google:gemini-embedding-001 (still available). Also correct the false claim that the model ID defaults to text-embedding-004: the runtime's google() embedding path requires an explicit model ID and errors with ModelNotProvided when absent (crates/runtime/src/model/embed.rs), across v1.11.x through trunk. Fixes #1926