Swap chapters 4 and 5

If we swap the chapters, we close https://github.com/purescript-contrib/purescript-book/issues/303 and partially address https://github.com/purescript-contrib/purescript-book/issues/340 and https://github.com/purescript-contrib/purescript-book/issues/212. There are no other dependencies between these chapters, so we get a simpler flow. Add a temporary note to these chapters not to confuse anyone actively working on them: ``` > Temporary note: If you're working on this chapter, beware that chapters 4 and 5 were swapped in November 2023. ```