Add solutions to intro and variables

solutions/01_variables/variables4.rs

@@ -1 +1,9 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn main() {
+    // In Rust, variables are immutable by default.
+    // Adding the `mut` keyword after `let` makes the declared variable mutable.
+    let mut x = 3;
+    println!("Number {x}");
+
+    x = 5; // Don't change this line
+    println!("Number {x}");
+}