Consistent use of singleton types in manual. (#36591)
Fixes #7135, see discussion there.
* separate discussion of singleton types (as in
`Base.issingletontype`) and `Type{T}`
* move discussion about singleton types to a later section so that
parametric singleton types can be discussed
* add example and discussion about parametric singleton types
* add usage hint about `Type` outside dispatch
Co-authored-by: Rafael Fourquet <fourquet.rafael@gmail.com>
Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
