Add instructions for accessing terminal in dom0 vs terminal in sd-admin

[ChumOfChance]

I've suggested elsewhere adding some introductory Qubes content: freedomofpress/securedrop-workstation-docs#344

Here I'm proposing a specific addition: with administration of SecureDrop being performed from the SecureDrop Workstation, an administrator now has to open two different terminals in two different qubes:

1. A terminal in dom0 for installing and configuring the SecureDrop Workstation itself and accessing logs
2. A terminal in sd-admin for performing administration on the servers.

I am assuming we will not be performing securedrop-admin actions and other installation steps from dom0 via qvm-run.

To someone not familiar with Qubes and navigating the Qubes menu, I think this could be a bit confusing. I think it would be useful to include instructions to explain the different between dom0 and sd-admin in brief, how to access the different terminals, how to tell which terminal you are in, etc.