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/22_clippy/clippy2.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let mut res = 42;
+    let option = Some(12);
+    // TODO: Fix the Clippy lint.
+    if let Some(x) = option {
+        res += x;
+    }
+
+    println!("{res}");
+}