Generalize USB creation instructions

[ChumOfChance]

Current state of documentation

The following pages cover the instructions for creating the individual Tails USB drives for Admin and Journalist Workstations:

1. Create USB Boot Drives (instructions for creating Tails and Ubuntu boot drives)
2. Set Up the Secure Viewing Station (describes the SVS Laptop airgap and adding a Tails Administration Password, and correcting Tails system time)
3. Set Up the Admin Workstation (starting with a Tails USB, add Tails Administrator Password, clone and verify SecureDrop repo, run tails-bootstrap)
4. Install SecureDrop (with all gathered values, run sdconfig and install from original Admin Workstation)
5. Configure the Admin Workstation (run localconfig)
6. Onboard Journalists (refers back to Tails part of 1. and 2. in entirety, with instructions to get v3 files and install before running localconfig)
7. Onboard Additional Admins (refers to 2. plus steps to transfer config files + secrets from existing Admin Workstation)

Describe the change

Page 1 should be split into separate pages for creating Tails USBs for SecureDrop and creating Ubuntu boot drives. Ubuntu boot drives are (ideally!) only created when first installing SecureDrop whereas Tails USBs might be created throughout a SecureDrop's life, to onboard new Journalists or Admins. The two sections do not share any content besides suggesting labeling USB drives and the total number of USB drives needed for a SecureDrop installation. Instructions for creating Ubuntu boot drives can be moved to the Set Up the Servers page.

Page 3 above is for setting up the initial Admin Workstation, but the instructions are functionally exactly the same as for a Journalist Workstation. This page should be tweaked so it is generic for creating a "SecureDrop USB" or "Workstation USB" (we can't say "SecureDrop Workstation USB").

If we make those changes, we will have generic Create Tails USB and Create SecureDrop USB (from Tails USB) pages, which are applicable to the SVS (Create Tails USB only) and all future Journalist and Admin Workstation USBs. We can make these generic instructions includes instead of standalone pages, so they can be included as needed throughout the documentation. Updating these docs will require changing only one place in the documentation source, rather than editing multiple pages.

How will this impact SecureDrop users?

There will be more repetition in the documentation, as the steps to create a Tails USB are repeated whenever they are invoked. A user will not have to jump around the documentation to accomplish a single task like creating a new Journalist USB (currently requires following parts of 3 different pages!).