docs: add OptStep API reference

[Haihan-Jiang]

Summary

• Adds jaxopt.OptStep to the API autosummary so solver return types can link to a generated reference page.
• Adds a short docstring for the OptStep container fields.

Fixes #251.

Testing

• python3.12 -m compileall -q jaxopt/_src/base.py
• git diff --check
• Temporary Python 3.12 venv smoke test with jax/jaxlib: imported jaxopt.OptStep and jaxopt.base.OptStep, verified they are the same object, and constructed an OptStep.

I did not run the full Sphinx docs build locally because docs/requirements.txt installs the full docs stack, including TensorFlow/Flax, for this small autosummary/docstring change.