Add descriptions/examples for advanced types

[gkepka]

Currently, the Other Types page of Scala 3 Book links to the language specification for literal and refined types. Descriptions for:

• higher-kinded types,
• literal/singleton types (why the different names?),
• refinement types,

are missing from Scala 3 Reference. Existential types are listed as a dropped feature and the description for kind polymorphism lacks an example.