docs: add scipy least squares API docs

[Haihan-Jiang]

Fixes #184.

What

• Add jaxopt.ScipyLeastSquares and jaxopt.ScipyBoundedLeastSquares to the API overview under nonlinear least squares.
• Add a short SciPy wrappers section to the nonlinear least squares docs so the classes have visible autosummary entries.

Testing

• git diff --check
• Temporary venv smoke test importing jaxopt.ScipyLeastSquares and jaxopt.ScipyBoundedLeastSquares with jax, jaxlib, and scipy installed.

The full Sphinx docs build was not run locally because the docs requirements install the full documentation stack, including TensorFlow/Flax.