docs: add documentation for Unit class

lib/src/unit.dart

@@ -1,4 +1,20 @@
-// ignore_for_file: unnecessary_new
+/// Represents a Unit type (a type with only one value).
+///
+/// In functional programming, Unit is used when a function
+/// needs to return a value, but the actual value is not important.
+///
+/// It is similar to `void`, but unlike `void`, it is a proper type
+/// and can be used in expressions.
+///
+/// Example:
+/// ```dart
+/// final u = unit;
+/// print(u); // ()
+/// ```
+///
+/// This is useful in cases where you want to indicate that
+/// a computation has completed but does not return meaningful data.
+/// // ignore_for_file: unnecessary_new
 // ignore_for_file: unnecessary_const
 
 part of dartz;