If you don't mind me asking, what in particular were you searching for? I agree that Nim's documentation needs a lot of love but would like suggestions on what specifically you had trouble with so that I can at least improve that (for now) :)
Sorry I can't be more specific.
I'm sure your original question was more nuanced, but for anyone wondering about this basically any ref type is heap allocated, and you do this by calling new, so:
type MyType = ref object
var myHeapObject: MyType
new myHeapObject
Pretty much anything else is stack allocated and doesn't need the GC if I remember right.