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/14_generics/README.md

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+# Generics
+
+Generics is the topic of generalizing types and functionalities to broader cases.
+This is extremely useful for reducing code duplication in many ways, but can call for some rather involved syntax.
+Namely, being generic requires taking great care to specify over which types a generic type is actually considered valid.
+The simplest and most common use of generics is for type parameters.
+
+## Further information
+
+- [Generic Data Types](https://doc.rust-lang.org/book/ch10-01-syntax.html)
+- [Bounds](https://doc.rust-lang.org/rust-by-example/generics/bounds.html)