Predefined type generators section

Again cover the basic:
- `keyof`
- Lookup `T[K]`
- Mapped
- Conditional (and infer)

And then cover the ones in `lib.d.ts` Pick / Readonly / Exclude / Return type etc.

Start by locating the book sections covering keyof, lookup types T[K], mapped types, and conditional types with infer. Review the predefined utility types named in lib.d.ts, including Pick, Readonly, Exclude, and ReturnType. Done means the basic generators and the listed library types are clearly explained in the appropriate documentation sections.

Written by the indexing model from the issue text.