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/02_functions/README.md

@@ -0,0 +1,8 @@
+# Functions
+
+Here, you'll learn how to write functions and how the Rust compiler can help you debug errors even
+in more complex code.
+
+## Further information
+
+- [How Functions Work](https://doc.rust-lang.org/book/ch03-03-how-functions-work.html)