Docs on Mooncake terminology (c.f. DI)

[yebai]

@gdalle wrote in #389 (comment)

In terms of terminology, I would change this to pushforward, mirroring DI conventions and your choice of pullback in reverse mode.

Essentially:

	general case	special case
forward mode	pushforward	derivative1 (pushforward of 1)
reverse mode	pullback	gradient (pullback of 1)

@willtebbutt replies

Ah, right. I chose this quite intentionally to be consistent with the docs. They discuss Frechet derivatives and their adjoints, and never use / define pushforward / pullback. I should probably actually change all uses of gradient and informal uses of the term pullback (eg. in rules in Mooncake) to be adjoint.

Agreed -- it makes sense to make the comparison clear. We've got the as-yet-to-be-written Mooncake.jl-API section of the tutorial in the docs, which will be the perfect home for it.

Footnotes

1. maybe not necessary in Mooncake itself, since DI takes care of it ↩