Add initial solution files for Rustlings exercises and quizzes

- Created empty solution files for various exercises in strings, modules, hashmaps, options, error handling, generics, traits, lifetimes, tests, iterators, smart pointers, threads, macros, clippy, conversions, and quizzes.
- Each solution file contains a main function with a comment indicating that it will be automatically filled after completing the exercise.
- Added a README.md file to provide information about the solutions and their purpose.

solutions/01_variables/variables5.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let number = "T-H-R-E-E";
+    println!("Spell a number: {number}");
+
+    // Using variable shadowing
+    // https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#shadowing
+    let number = 3;
+    println!("Number plus two is: {}", number + 2);
+}