Update the tools doc

##### Description

I updated the tools doc as part of a general review of the Apps reference docs. I'm proposing a fairly substantial overhaul of this document. Here's the rundown:

- Added a bit more info about the tools in the intro.

- Replaced the installation and some of the usage info with a link to the more comprehensive instructions at [Installing and using the Zendesk apps tools](https://support.zendesk.com/entries/25079228-Installing-and-using-the-Zendesk-apps-tools). Also included a link to the new [ZAT known issues](https://support.zendesk.com/entries/42600698-Zendesk-app-tools-Known-issues-) doc.

- Recast the content to be a command reference more in line with the reference guide that contains it. 

@zendesk/quokka
@maximeprades