Draft: WIP: Expand the haddocks for Control.Category

The documentation for Control.Category in base is very sparse currently, and rather meaningless a lot if you don't already know what a category is. This MR expands it a little, adding a basic explanation and some examples. There's definitely more to improve here, but this should already be a step forward. (Although of course I wouldn't want this module to eventually contain a complete introduction to category theory.)

• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)
• mentions new features in the release notes for the next release