Update the Quickstart guide

Nobody has claimed this yet.

Starting at the "Create a staging environment", the steps described in the guide quickly outpace the explanations and it was not obvious to me what was going on. The quickstart assumes knowledge of concepts (environments, machines) that I would have liked to see brief explanations of.

The Quickstart could also use now.sh as an example deployment environment so that the user can follow along with the guide themselves. I could follow along almost exactly with the first half of the guide, but after "Take secrets beyond development" I couldn't follow along myself.

Start with the Quickstart guide at “Create a staging environment” and read through “Take secrets beyond development” to identify where explanations are missing. Add brief context for environments and machines, include now.sh as a deployment example, and verify that readers can follow the guide through the later steps.

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