Add speaker notes to Fibonacci exercise (#2898)

The speaker notes should prompt a deeper discussion around the
exercise by calling out the non-obvious integer overflow pitfall and
encourage instructors to explore alternative implementations with
students, moving beyond a surface-level recursive solution.

src/types-and-values/exercise.md

@@ -23,3 +23,15 @@ this function panic?
 
 {{#include exercise.rs:main}}
 ```
+
+<details>
+
+- This exercise is a classic introduction to recursion.
+- Encourage students to think about the base cases and the recursive step.
+- The question "When will this function panic?" is a hint to think about integer
+  overflow. The Fibonacci sequence grows quickly!
+- Students might come up with an iterative solution as well, which is a great
+  opportunity to discuss the trade-offs between recursion and iteration (e.g.,
+  performance, stack overflow for deep recursion).
+
+</details>

src/types-and-values/solution.md

@@ -3,3 +3,21 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+<details>
+
+- Walk through the solution step-by-step.
+- Explain the recursive calls and how they lead to the final result.
+- Discuss the integer overflow issue. With `u32`, the function will panic for
+  `n` around 47. You can demonstrate this by changing the input to `main`.
+- Show an iterative solution as an alternative and compare its performance and
+  memory usage with the recursive one. An iterative solution will be much more
+  efficient.
+
+## More to Explore
+
+For a more advanced discussion, you can introduce memoization or dynamic
+programming to optimize the recursive Fibonacci calculation, although this is
+beyond the scope of the current topic.
+
+</details>