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/06_move_semantics/move_semantics5.rs

@@ -0,0 +1,24 @@
+#![allow(clippy::ptr_arg)]
+
+// TODO: Fix the compiler errors without changing anything except adding or
+// removing references (the character `&`).
+
+// Shouldn't take ownership
+fn get_char(data: &String) -> char {
+    data.chars().last().unwrap()
+}
+
+// Should take ownership
+fn string_uppercase(mut data: String) {
+    data = data.to_uppercase();
+
+    println!("{data}");
+}
+
+fn main() {
+    let data = "Rust is great!".to_string();
+
+    get_char(&data);
+
+    string_uppercase(data);
+}