RFC: update jltypes reference diagram for the current v1.6 type system

[vtjnash]

Attempted updated of the old type-system categorization diagram to show some of the intent with the current system: https://github.com/JuliaLang/julia/blob/0b5777f1cd0ed1cd193eee08192412d07b2f1ecc/doc/src/assets/jltypes.svg

(somewhat inspired by #25796)

[JeffBezanson]

Found a couple issues:

• The isa arrows from UnionAll and Union to DataType are pointing the wrong direction
• I would not connect Union and Union{}. Instead put TypeofBottom next to Union.
• isa arrow from Union{} to TypeofBottom goes the wrong way
• For the criteria for when an immutable is boxed, I would say - might have undefined fields, - is self-referential, or - has too many pointers.

[vtjnash]

While I think this was a helpful exercise, I think we don't need this anymore