Add link to Math object in Freelancer Rates intro

[StreakInTheSky]

Since the exercise used for Numbers and Arithmetic Operators is combined, if the user were to come into it from Arithmetic Operations without reading the Numbers concept, they would not be introduced to the Math object, which is necessary to complete the exercise. It's mentioned in the hints, but I don't think it's enough for someone new to JS.

I tried to use the phrasing and structure of the related docs, but let me know if there's a better way to do it.

[github-actions]

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[junedev]

@StreakInTheSky Someone is currently working on cleaning up freelancer rates, see #1049. Part of that is also to point to the built-in math object for the rounding. (Its one of the items in the todo list there). I don't want to make things difficult for the person working on this so I will close this PR and let them do their work instead. Hope this is ok for you.

As for the math package in general, there will be a separate exercise on this in the future.

Regarding you concern about "coming from arithmetic operators": When you actually start the exercise, you see an introduction document that includes the content of all the introductions of the concepts that are taught by this exercise. So it doesn't matter from which concept you started.

[StreakInTheSky]

Oh ok, sounds good.

About your last point, the Math object isn’t actually linked in the exercise which is what I was adding. But looks like that will be fixed.