add documentation for single-file Haskell scripts (#8559)

* add documentation for single-file Haskell scripts

This is a useful feature which I think would benefit newcomers by being a top-level, early entry in the docs. See: https://stackoverflow.com/a/65541020

* Add project metadata and cabal run doc link

* Fix cabal run link

* Add custom anchor for 'cabal run'

* Qualify use of executable shell script to unix-likes