Add an instruction on how to format code with Prettier

[Droid-An]

Which module(s) and week(s) does this change affect?

Module(s): Onboarding
Week(s): 1

What is the work that needs to be done?

Adding a short explanation on how to format code with Prettier in the onboarding module.

Why is this work important to do?

It will give learners and trainees an understanding of how to use this extension and help volunteers review pull requests.

Additional context

learners and trainees