Closes JuliaLang#43811.

- The intro section felt a bit long-winded, I've made some changes there first.
- Clarify that `typeof` returns the concrete type
- Rename Type Declarations section to Type Annotations,
  avoid confusion with Declaring Types section and distinguish use of "type declaration"
  to mean "declaring new types"
- Removed some of the jargon and wikipedia links to make room for
  a brief alternative `Point{T1,T2}` demonstration.
- Shifted some paragraphs around to reflect their importance,
  and changed the wording in some places.
- Rename type declaration -> annotation in other places (docstrings/comments)

Co-authored-by: adienes <51664769+adienes@users.noreply.github.com>

Co-authored-by: adienes <51664769+adienes@users.noreply.github.com>