user's guide: consolidate defaulting documentation

This commit adds a new section on defaulting, which consolidates various
parts of documentation surrounding defaulting into one central place.

It explains type class defaulting in detail, extensions to it with
OverloadedStrings, NamedDefaults and ExtendedDefaultRules, as well
as other defaulting mechanisms (e.g. kind-based defaulting such as
RuntimeRep defaulting, and defaulting of equalities).