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/clippy1.rs

@@ -0,0 +1,15 @@
+// The Clippy tool is a collection of lints to analyze your code so you can
+// catch common mistakes and improve your Rust code.
+//
+// For these exercises, the code will fail to compile when there are Clippy
+// warnings. Check Clippy's suggestions from the output to solve the exercise.
+
+fn main() {
+    // TODO: Fix the Clippy lint in this line.
+    let pi: f32 = std::f32::consts::PI;
+    let radius: f32 = 5.0;
+
+    let area = pi * radius.powi(2);
+
+    println!("The area of a circle with radius {radius:.2} is {area:.5}");
+}