Add “More to Explore” sections (#1369)

From a discussion in #1313.

src/memory-management/stack.md

@@ -29,22 +29,25 @@ fn main() {
 
 * If students ask about it, you can mention that the underlying memory is heap allocated using the [System Allocator] and custom allocators can be implemented using the [Allocator API]
 
-* We can inspect the memory layout with `unsafe` code. However, you should point out that this is rightfully unsafe!
+## More to Explore
 
-    ```rust,editable
-    fn main() {
-        let mut s1 = String::from("Hello");
-        s1.push(' ');
-        s1.push_str("world");
-        // DON'T DO THIS AT HOME! For educational purposes only.
-        // String provides no guarantees about its layout, so this could lead to
-        // undefined behavior.
-        unsafe {
-            let (ptr, capacity, len): (usize, usize, usize) = std::mem::transmute(s1);
-            println!("ptr = {ptr:#x}, len = {len}, capacity = {capacity}");
-        }
+We can inspect the memory layout with `unsafe` Rust. However, you should point
+out that this is rightfully unsafe!
+
+```rust,editable
+fn main() {
+    let mut s1 = String::from("Hello");
+    s1.push(' ');
+    s1.push_str("world");
+    // DON'T DO THIS AT HOME! For educational purposes only.
+    // String provides no guarantees about its layout, so this could lead to
+    // undefined behavior.
+    unsafe {
+        let (ptr, capacity, len): (usize, usize, usize) = std::mem::transmute(s1);
+        println!("ptr = {ptr:#x}, len = {len}, capacity = {capacity}");
     }
-    ```
+}
+```
 
 </details>