doc: documentating every helpers of "it" sub-package

docs/data/it-countvaluesby.md

@@ -0,0 +1,35 @@
+---
+name: CountValuesBy
+slug: countvaluesby
+sourceRef: it/seq.go#L720
+category: it
+subCategory: slice
+signatures:
+  - "func CountValuesBy[T any, U comparable](collection iter.Seq[T], transform func(item T) U) map[U]int"
+variantHelpers: []
+similarHelpers:
+  - core#slice#countvaluesby
+position: 204
+---
+
+CountValuesBy counts the number of each element returned from transform function.
+Is equivalent to chaining Map and CountValues.
+
+```go
+type Person struct {
+    Name string
+    Age  int
+}
+
+collection := func(yield func(Person) bool) {
+    yield(Person{"Alice", 25})
+    yield(Person{"Bob", 30})
+    yield(Person{"Charlie", 25})
+    yield(Person{"Diana", 30})
+}
+
+countsByAge := it.CountValuesBy(collection, func(p Person) int {
+    return p.Age
+})
+// countsByAge contains {25: 2, 30: 2}
+```