Add tut doc for FunctionK (#1604)

* Add tut doc for FunctionK

I'm sure that there is more that could be done with this, and I'm very
open to recommendations, but I wanted to get something out there.

* FunctionK doc improvements

- Clean up sloppy usage of the term "kind"
- Mention the `~>` type alias
- Show example of fixing the left of `Either` to make it work with
`FunctionK`.

* Add site menu link for FunctionK docs

docs/src/main/resources/microsite/data/menu.yml

@@ -124,6 +124,10 @@ options:
     url: datatypes/freemonad.html
     menu_type: data
 
+  - title: FunctionK
+    url: datatypes/functionk.html
+    menu_type: data
+
   - title: Id
     url: datatypes/id.html
     menu_type: data

docs/src/main/tut/datatypes/functionk.md

@@ -0,0 +1,123 @@
+---
+layout: docs
+title:  "FunctionK"
+section: "data"
+source: "core/src/main/scala/cats/arrow/FunctionK.scala"
+scaladoc: "#cats.arrow.FunctionK"
+---
+# FunctionK
+A `FunctionK` transforms values from one first-order-kinded type (a type that takes a single type
+parameter, such as `List` or `Option`) into another first-order-kinded type. This transformation is
+universal, meaning that a `FunctionK[List, Option]` will translate all `List[A]` values into an
+`Option[A]` value for all possible types of `A`. This explanation may be easier to understand if we
+first step back and talk about ordinary functions.
+
+## Ordinary Functions
+Consider the following scala method:
+
+```tut:silent
+def first(l: List[Int]): Option[Int] = l.headOption
+```
+
+This isn't a particularly helpful method, but it will work as an example. Instead of writing this as a
+method, we could have written this as a function _value_:
+
+```tut:silent
+val first: List[Int] => Option[Int] = l => l.headOption
+```
+
+And here, `=>` is really just some syntactic sugar for `Function1`, so we could also write that as:
+
+```tut:silent
+val first: Function1[List[Int], Option[Int]] = l => l.headOption
+````
+
+Let's cut through the syntactic sugar even a little bit further. `Function1` isn't really a special type.
+It's just a trait that looks something like this:
+
+```tut:silent
+// we are calling this `MyFunction1` so we don't collide with the actual `Function1`
+trait MyFunction1[A, B] {
+  def apply(a: A): B
+}
+```
+
+So if we didn't mind being a bit verbose, we could have written our function as:
+
+```tut:silent
+val first: Function1[List[Int], Option[Int]] = new Function1[List[Int], Option[Int]] {
+  def apply(l: List[Int]): Option[Int] = l.headOption
+}
+```
+
+## Abstracting via Generics
+
+Recall our `first` method:
+
+```tut:silent
+def first(l: List[Int]): Option[Int] = l.headOption
+```
+
+The astute reader may have noticed that there's really no reason that this method needs to be tied directly to `Int`. We could use generics to make this a bit more general:
+
+```
+def first[A](l: List[A]): Option[A] = l.headOption
+```
+
+But how would we represent this new `first` method as a `=>`/`Function1` value? We are looking for something like a type of `List[A] => Option[A] forAll A`, but this isn't valid scala syntax. `Function1` isn't quite the right fit, because its `apply` method doesn't take a generic type parameter.
+
+## Higher Kinds to the Rescue
+
+It turns out that we can represent our universal `List` to `Option` transformation with something that looks a bit like `Function1` but
+that adds a type parameter to the `apply` method and utilizes higher kinds:
+
+```tut:silent
+trait MyFunctionK[F[_], G[_]] {
+  def apply[A](fa: F[A]): G[A]
+}
+```
+
+Cats provides this type as `FunctionK` (we used `MyFunctionK` for our example type to avoid confusion). So now we can write `first` as a `FunctionK[List, Option]` value:
+
+```tut:silent
+import cats.arrow.FunctionK
+
+val first: FunctionK[List, Option] = new FunctionK[List, Option] {
+  def apply[A](l: List[A]): Option[A] = l.headOption
+}
+```
+
+## Syntactic Sugar
+
+If the example above looks a bit too verbose for you, the [kind-projector](https://github.com/non/kind-projector)
+compiler plugin [provides](https://github.com/non/kind-projector#polymorphic-lambda-values) a more concise syntax.
+After adding the plugin to your project, you could write the `first` example as:
+
+```tut:silent
+val first: FunctionK[List, Option] = λ[FunctionK[List, Option]](_.headOption)
+```
+
+Cats also provides a `~>` type alias for `FunctionK`, so an even more concise version would be:
+
+```tut:silent
+import cats.~>
+
+val first: List ~> Option = λ[List ~> Option](_.headOption)
+```
+
+Being able to use `~>` as an alias for `FunctionK` parallels being able to use `=>` as an alias for `Function1`.
+
+## Use-cases
+
+`FunctionK` tends to show up when there is abstraction over higher-kinds. For example, interpreters for [free monads](freemonad.html) and [free applicatives](freeapplicative.html) are represented as `FunctionK` instances.
+
+## Types with more than one type parameter
+
+Earlier it was mentioned that `FunctionK` operates on first-order-kinded types (types that take a single type parameter such as `List` or `Option`). It's still possible to use `FunctionK` with types that would normally take more than one type parameter (such as `Either`) if we fix all of the type parameters except for one. For example:
+
+```tut:silent
+type ErrorOr[A] = Either[String, A]
+
+val errorOrFirst: FunctionK[List, ErrorOr] =
+  λ[FunctionK[List, ErrorOr]](_.headOption.toRight("ERROR: the list was empty!"))
+```