docs: add typed models concept guide (#777)

## Summary

Adds a concepts page documenting the v3 typed-model surface: how
response Pydantic models are accessed, how to provide structured input
as plain dicts (with matching TypedDicts for shape), forward
compatibility under `extra='allow'`, and where to browse all generated
models in the API reference.

The README's `Fully typed responses` feature bullet referenced typed
models without a dedicated concept page — this fills that gap.