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.

exercises/01_variables/variables5.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let number = "T-H-R-E-E"; // Don't change this line
+    println!("Spell a number: {number}");
+
+    // TODO: Fix the compiler error by changing the line below without renaming the variable.
+    let number = 3;
+    println!("Number plus two is: {}", number + 2);
+}