docs: Create test zone in quick start

[thgoebel]

Instruct users how to create test zone in the quick start. This makes it easier to follow the quick start if you don't have a zone file lying around.

The HSM guides (SoftHSM, SmartCard-HSM, NetHSM) all have a "Sign a Test Zone" section. This shows that there is a user need for this piece of information. We might as well move this up to the generic quick start guide.

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