Add introductory section "Why k6"

[MattDodsonEnglish]

At the broadest perspective, all load-test applications do more or less the same thing: they generate concurrent load and present metrics. Of course, there's a substantial amount of difference in the details of implementation.

Perhaps the workshops could make a factual, non-market-y case for why k6 is a good choice for load testing. It may help people see the "so what" behind the workshop. What's more, this exercise could perhaps carry over to other parts of the docs.