Improve "Setting up" documentation

[michaelw85]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

I've followed this guide to set up a dev environment to make my first code contribution. I found it somewhat confusing, I was missing core repo's, ended up installing them with the wp package command which affected my existing WP CLI install. I asked for input on Slack on my setup and followed different instructions to setup my dev environment which was easy and resulted in a better setup in my opinion.

Setting-up page

Describe the solution you'd like

I would suggest using wp cli dev setup.
If this is overkill for a small contribution, I would add an explanation it is also possible to fork and clone a single repo (is there a list?).