Docs updates (#198)

Reorganize NN docs + a few other tidbits.

docs/src/python/nn/functions.rst

@@ -0,0 +1,23 @@
+.. _nn_functions:
+
+.. currentmodule:: mlx.nn
+
+Functions
+---------
+
+Layers without parameters (e.g. activation functions) are also provided as
+simple functions.
+
+.. autosummary::
+   :toctree: _autosummary_functions
+   :template: nn-module-template.rst
+
+   gelu
+   gelu_approx
+   gelu_fast_approx
+   relu
+   prelu
+   silu
+   step
+   selu
+   mish