Consolidate documentation of defaulting in the user's guide
This MR consolidates the documentation in the user's guide relating to (top-level) defaulting. Ticket: #25807 (closed).
Due to a large amount of spam we do not allow new users to create repositories, they are "external" users. If you are a new user and want to create a repository, for example for forking GHC, open a new issue on ghc/ghc using the "get-verified" issue template
This MR consolidates the documentation in the user's guide relating to (top-level) defaulting. Ticket: #25807 (closed).