Leveling Up - Generating documentation with haddock

file to edit: https://github.com/haskell/cabal-userguide/blob/main/src/leveling_up/05_generating_documentation_with_haddock.md

## Brief Summary

In my opinion this is a relatively undocumented aspect of cabal. I think that it can be very mysterious to beginners, it is also really useful to be able to locally host project specific haddocks. It can also be confusing when misconfigured i.e. you are building haddocks on every build / ghcid recompilation. While this shouldn't be a haddock reference guide, it might be useful to cover some of the basic / more common haddock syntax in the example.