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/01_variables/README.md

@@ -0,0 +1,9 @@
+# Variables
+
+In Rust, variables are immutable by default.
+When a variable is immutable, once a value is bound to a name, you can't change that value.
+You can make them mutable by adding `mut` in front of the variable name.
+
+## Further information
+
+- [Variables and Mutability](https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html)