[docs/base] add a type tree for `Number` (#45975)
* [docs/base] add a type tree
https://github.com/JuliaLang/julia/issues/24741
Generated using the following code with manual modifications
```jl
using AbstractTrees
AbstractTrees.children(x::Type) = subtypes(x)
print_tree(Number)
```
* [docs/base] mark "Abstract Type"
* [docs/base] add a blurb for the tree
* Update numbers.md
Co-authored-by: Kristoffer Carlsson <kcarlsson89@gmail.com>
