message: Move documentation on types-of-types to separate page
diff: - [Language](./language/index.md)
  - [REPL](./language/repl.md)
  - [Type inference](./language/type-inference.md)
  + - [Types of types](./language/types-of-types.md)
  - [Appendix](./appendix/index.md)
  - [Design](./appendix/design.md)
  - [Theory](./appendix/theory.md)
  