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/02_functions/functions5.rs

@@ -0,0 +1,9 @@
+// TODO: Fix the function body without changing the signature.
+fn square(num: i32) -> i32 {
+    num * num
+}
+
+fn main() {
+    let answer = square(3);
+    println!("The square of 3 is {answer}");
+}