Improve Concept Exercise: Freelancer Rates

[JaPrad]

As discussed in #1049, in this PR I've:

1. Rearranged and simplified tasks for the ease of learning and updated files accordingly
2. Introduced the use of remainder operator and modified design.md accordingly
3. Shown the use of Math.ceil and Math.floor in introduction.md

[JaPrad]

Thank you for reviewing @junedev! I've incorporated all of your suggestions.

[junedev]

@JaPatGitHub Thanks a lot for your ongoing effort to improve this exercise!

The rounding section still had some typos that I fixed and I changed it so the link is on rounding and not on Math. In about.md I moved the content to the "Built-In object" section since Math is already mentioned there. See 98c7a70. Hope thats ok for you.

[SleeplessByte]

(love this!!)

[JaPrad]

My pleasure to work with you guys on this!