Document optimizer rules

[chexxor]

Some PS functions are optimized away, like (<<<) and ($). We should add a docpage which introduces the optimizer and some important things to know it does, as a PureScript user.

I've avoided using ($) because I thought it added another layer to the stack for each use, but I'll use it more after learning that PSC optimizes it away.

[chexxor]

Looks like a list of the notable inlined functions is here: https://github.com/purescript/purescript/blob/0f43bb6ccb9f6ed1411f95e4fbe58801cdb21a8e/src/Language/PureScript/CoreImp/Optimizer/Inliner.hs#L111