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/09_strings/strings2.rs

@@ -0,0 +1,14 @@
+// TODO: Fix the compiler error in the `main` function without changing this function.
+fn is_a_color_word(attempt: &str) -> bool {
+    attempt == "green" || attempt == "blue" || attempt == "red"
+}
+
+fn main() {
+    let word = String::from("green"); // Don't change this line.
+
+    if is_a_color_word(&word) {
+        println!("That is a color word I know!");
+    } else {
+        println!("That is not a color word I know.");
+    }
+}