Add a "gradient zoo" page?

[mcabbott]

A while back I tried to write up a docs page pointing out that Flux is AD-agnostic -- anything which produces an appropriately nested structure should work.

This is a rough draft, help with tidying it up would be welcome.

Perhaps it's too opinionated about other packages. Certainly any opinions it offers should have clear dates attached.

Rendered version: https://github.com/mcabbott/Flux.jl/blob/grad_zoo/docs/src/tutorials/gradient_zoo.md