docs: update concurrency docs and examples to use []thread{} (#8933)

examples/concurrency/concurrency.v

@@ -1,21 +1,18 @@
-import sync
 import time
 
 // Simulate expensive computing using sleep function
-fn expensive_computing(id int, duration int, mut wg sync.WaitGroup) {
+fn expensive_computing(id int, duration int) {
 	println('Executing expensive computing task ($id)...')
 	time.wait(duration * time.millisecond)
 	println('Finish task $id on $duration ms')
-	wg.done()
 }
 
 fn main() {
-	mut wg := sync.new_waitgroup()
-	wg.add(3)
-	go expensive_computing(1, 100, mut wg)
-	go expensive_computing(2, 500, mut wg)
-	go expensive_computing(3, 1000, mut wg)
+	mut threads := []thread{}
+	threads << go expensive_computing(1, 100)
+	threads << go expensive_computing(2, 500)
+	threads << go expensive_computing(3, 1000)
 	// Join all tasks
-	wg.wait()
+	threads.wait()
 	println('All jobs finished!')
 }

examples/concurrency/concurrency_returns.v

@@ -0,0 +1,13 @@
+fn expensive_computing(i int) int {
+	return i * i
+}
+
+fn main() {
+	mut threads := []thread int{}
+	for i in 1 .. 10 {
+		threads << go expensive_computing(i)
+	}
+	// Join all tasks
+	r := threads.wait()
+	println('All jobs finished: $r')
+}