docs: add a recursive structure and binary tree example in the sumtype section (#8122)

doc/docs.md

@@ -1884,6 +1884,35 @@ println(sum)
 The built-in method `type_name` returns the name of the currently held
 type.
 
+With sum types you could build recursive structures and write concise but powerful code on them.
+```v
+// V's binary tree
+struct Empty {}
+
+struct Node {
+	value f64
+	left  Tree
+	right Tree
+}
+
+type Tree = Empty | Node
+
+// sum up all node values
+fn sum(tree Tree) f64 {
+	return match tree {
+		Empty { f64(0) } // TODO: as match gets smarter just remove f64()
+		Node { tree.value + sum(tree.left) + sum(tree.right) }
+	}
+}
+
+fn main() {
+	left := Node{0.2, Empty{}, Empty{}}
+	right := Node{0.3, Empty{}, Node{0.4, Empty{}, Empty{}}}
+	tree := Node{0.5, left, right}
+	println(sum(tree)) // 0.2 + 0.3 + 0.4 + 0.5 = 1.4
+}
+```
+
 #### Dynamic casts
 
 To check whether a sum type instance holds a certain type, use `sum is Type`.