Upgrade to SnakeBridge 0.13.0 which extracts Python docstrings into
generated @moduledoc/@doc attributes. Adds ExDoc integration via
SnakeBridge.Docs helpers to group and nest Dspy.* modules by their
original DSPy package structure.

- Regenerate all DSPy bindings with docstring metadata
- Add snakebridge_groups/snakebridge_nests helpers for HexDocs
- Update README to describe dual API (generated modules + FFI wrapper)
- Bump minimum Python version to 3.9+