feat(conversions): add hint comments

exercises/conversions/from_into.rs

@@ -1,6 +1,8 @@
 // The From trait is used for value-to-value conversions.
 // If From is implemented correctly for a type, the Into trait should work conversely.
 // You can read more about it at https://doc.rust-lang.org/std/convert/trait.From.html
+// Execute `rustlings hint from_into` or use the `hint` watch subcommand for a hint.
+
 #[derive(Debug)]
 struct Person {
     name: String,