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/variables6.rs

@@ -0,0 +1,6 @@
+// TODO: Change the line below to fix the compiler error.
+const NUMBER: i32 = 3;
+
+fn main() {
+    println!("Number: {NUMBER}");
+}