Add softsign to the nn functions docs

[ayaangazali]

Proposed changes

nn.softsign is missing from the neural network functions list, so it has no page in the docs. The Softsign layer is documented and its docstring points at the function:

See :func:softsign for the functional equivalent.

Since the function has no target, that link is dead. Building with -n shows it:

python/mlx/nn/layers/activations.py:docstring of mlx.nn.layers.activations.Softsign:3:
WARNING: py:func reference target not found: softsign [ref.func]

Every other activation in that file has its functional form listed, including the neighbours softmax, softmin, softplus and softshrink, so this looks like it was just skipped. Adding the entry generates mlx.nn.softsign.rst and the warning goes away.

>>> nn.softsign(mx.array([-1.0, 0.0, 1.0]))
array([-0.5, 0, 0.5], dtype=float32)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

(docs only, one line; verified by building the docs before and after)

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as usual, being straight about process: freshman contributor, Claude Code helps me sweep, but i found this by building the docs in nitpick mode, read the actual warning, and checked that every sibling activation is listed before adding it.