chore: add resource under cpp:introduction

src/data/roadmaps/cpp/content/100-introduction/index.md

@@ -128,3 +128,5 @@ int main() {
 ```
 
 This basic introduction to C++ should provide you with a good foundation for further learning. Explore more topics such as classes, objects, inheritance, polymorphism, templates, and the Standard Template Library (STL) to deepen your understanding of C++ and start writing more advanced programs.
+
+- [LearnC++](https://www.learncpp.com/)