Add content on Overlapping Instances #358
Description
Related to purescript-contrib/purescript-book#242 which proposes moving the book's section on Overlapping Instances to one or more of these locations in this docs repo:
- Type classes reference
- Guides section
- Error explanation for overlapping instances, or at least edit the error to link back to a more thorough explanation (some goes for the Orphan Instances error)