Avoid kind-projector in docs where possible

[ceedubs]

The kind-projector usage in the docs trips up a lot of people (as evident by it frequently coming up in Gitter questions). I think that where possible we should use simple type aliases instead of kind-projector syntax.

If we have any cases where a simple type alias isn't possible, I like using kind-projector instead of a type lambda, but I think that we should preface the first such usage on each page with an explanation that kind-projector is being used.

[peterneyens]

In the Functor documentation there already is an explanation about the kind projector plugin.

Do you suggest writing something similar on the other pages where kind projector usage can't be replaced by a type alias, or should we create a new page about kind projector where we could link to ?

[ceedubs]

@peterneyens I guess it would depend on how many can't be replaced by a simple type alias. If it's only one then inline is probably fine, but otherwise we should probably create a new page. Either way having a new page that we link to probably wouldn't be a bad thing.

[ceedubs]

I'm working on this.