Document instances of type classes

I think it would be very nice to have list of instances for which type class is implemented on type class documentation page. For example, on Functor page we can list:

• Option
• List
• Stream
• Future
• Xor
• etc

I like that we have this on Hackage, but for cats this probably should be done manually, (although I think I heard something about support of type classes in scaladoc) to concisely highlight implementation details and just because I doubt this can be autogenerated.

I think this can highly improve approachability in following ways:

• Inductive reasoning. It's just easier to understand new concepts by example. Example instances are what newcomers want to see to draw lines between unconnected at first glance entities
• While some instances like Option and List for Functor are obvious and well-known, some are not (functions). At least they weren't for me
• For people migrating from scalaz or Haskell it is good to know for what instances are implemented and what aren't without looking at code (like in Hackage, again)
• It also can be a good idea to describe how instances are defined. For example it can be not obvious at first glance why haven't with Monad[Option] in optionInstances

Start with the Functor documentation page linked in the issue and inspect the other type-class documentation pages for their current structure. Determine which implemented instances should be listed and how instance definitions should be described; done means the relevant documentation consistently presents this information without relying on undocumented assumptions about generated scaladoc.