Introduce tangent and cotangents terminology

[mzgubic]

We introduce the terms tangent and cotangent very briefly in the introduction page. We should probably have an accessible introduction in the maths section (without using too much maths).

[oxinabox]

cf #117

[ekinakyurek]

I am here just to say that I really enjoyed reading the documentation! Specifically deriving array rulesand propagators sections are super nice. Thank you for doing this

PS: I think the only part I am kind of lost is formal definitions under here, it was a bit sudden for me: https://juliadiff.org/ChainRulesCore.jl/dev/maths/propagators.html#Geometric-interpretation-of-reverse-and-forwards-mode-AD