Add solutions to intro and variables

solutions/01_variables/variables3.rs

@@ -1 +1,13 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn main() {
+    // Reading uninitialized variables isn't allowed in Rust!
+    // Therefore, we need to assign a value first.
+    let x: i32 = 42;
+
+    println!("Number {x}");
+
+    // It possible to declare a variable and initialize it later.
+    // But it can't be used before initialization.
+    let y: i32;
+    y = 42;
+    println!("Number {y}");
+}