JavaScript: Add a mentoring note to the Triangle exercise

As the title we need a mentoring note to the Triangle exercise.

Of course, there are other exercises that doesn't have a mentoring note and I think they should also have, but I think Triangle is the top priority for the following reasons:

- this is the one of the four side exercises to be unlocked at first and students' submissions are many
- mentors are reluctant to mentor this and students' submissions are always queued