Add high-level documentation

[milesfrain]

This issue was raised in a slack conversation:

where is it documented what Prelude is
I just started and I see Prelude import as the first thing in most file
but I cant find a documentation that explains that

Looks like it's only mentioned here with the hope that readers will jump to the appropriate conclusions about what it is, although I agree that's not ideal. https://github.com/purescript/documentation/blob/master/guides/Getting-Started.md
It would be better to add some details here: https://pursuit.purescript.org/packages/purescript-prelude/4.1.1
or here:
https://pursuit.purescript.org/packages/purescript-prelude/4.1.1/docs/Prelude