Update Frontend JS Testing Documentation

What does this MR do?

I went over the JS documentation, and found lots of issues with the state of today. I put more emphasis on the fact that we move to Jest and made it more clear as part of the intro.

With that in mind I also regrouped some parts to be logically together. You'll also find some extras added, and especially a bigger section around how and what to test, with references to parts further down.

There are more changes to come, but as a first step this is what I wanted to add.