Add a note about prerequesites to the Journalist guide

[nathandyer]

Describe the change

Right now, the Journalist guide assumes you're ready to jump right into interacting with the Journalist Interface: https://docs.securedrop.org/en/stable/journalist.html

We should add a note up-front that this guide assumes SecureDrop has already been installed, and that the Journalist has been provisioned with a Journalist Workstation USB by the Admin.