jax.scipy.special: manually document functions to avoid scipy import

[jakevdp]

This lets us give more implementation-specific information, and lets us avoid a dependency on scipy. Related to #20799.

Preview rendered docstrings here: https://jax--20822.org.readthedocs.build/en/20822/jax.scipy.html#module-jax.scipy.special

Part of #21461.

[hawkinsp]

Same general comments on these:

• use type links where possible for Array and ArrayLike
• at least link to definition of the math in each case (Wikipedia being good), although having formulae is better where we can.
• an example would be peachy.

Doable?

[jakevdp]

It's all doable, it's just about how much time we want to invest

use type links where possible for Array and ArrayLike

This is done automatically by sphinx; e.g. https://jax--20822.org.readthedocs.build/en/20822/_autosummary/jax.scipy.special.bernoulli.html#jax.scipy.special.bernoulli

at least link to definition of the math in each case

I linked to the numpy documentation in each case, which generally includes the math. Is that sufficient?

[jakevdp]

Mathematical definitions added