Better introduction to parametric types in the manual

After a discussion in IRC today I realised that there were a few things about parametric types that aren't easily understood. It seems that the manual could help a bit more in introducing parametric types. I'm opening this issue to outline some of the things I think could be documented better, and also to gather ideas or examples that might help to write a more hands-on parametric types section in the manual. Not sure if this belongs on the forum but I am thinking of linking a PR at some point.

For the parametric type Foo and an instance foo, I think the docs could clarify

• why both isabstracttype(Foo) and isconcretetype(Foo) return false for the parametric type Foo, i.e. that parametric types are neither abstract nor concrete
• the difference between the following definitions of Foo

struct Foo{T}
    a::T
    b::T
end

struct Foo{T1, T2}
    a::T1
    b::T2
end

• why typeof(foo) <: Foo is true, however supertype(typeof(foo)) == Foo is false (it's Any)
• why Foo isa DataType is false

and (more visibly) inform

• that parametric types cannot be subtyped they can, parametric abstract types are a thing
• that reified instances of parametric types are of a concrete type, therefore typeof(foo) != Foo where foo is an instance of Foo
• that foo isa Foo must be used to test if foo is an instance of Foo

I also think there should be one example on how to set up a simple but complete type hierarchy (without going into all of the gritty details of the Advanced Types section). Something like the comment by wasshin in #4935.

There might be some other things I've forgotten, feel free to suggest related improvements. I feel that parametric types are one of the major features of Julia's type system, however they can be confusing when coming from languages with more traditional/simple OOP systems. Understanding parametric types should help to reduce common antipatterns like over-constraining argument types, and help people to leverage the dispatch system (what are all those <: needed for anyway?)