Update index.md: Article about function pointers was added (#5916)

I added the link to the article about function pointers in C++ that helped me to learn more about it and try it on my own.

src/data/roadmaps/cpp/content/105-pointers-and-references/index.md

@@ -54,3 +54,7 @@ int &ref = num; // Reference 'ref' is now an alias of 'num'
 Modifying the value of `ref` will also modify the value of `num` because they share the same memory location.
 
 **Note:** References are generally used when you want to pass a variable by reference in function arguments or when you want to create an alias for a variable without the need for pointer syntax.
+
+Learn more from the following resources:
+
+- [@article@Function Pointer in C++](https://www.scaler.com/topics/cpp/function-pointer-cpp/)