Docs: transfer onion address

[jskinne3]

The docs lack clear instructions on how to configure the onion hostname without manual typing of a long, random string which is prone to error. Docs should mention a method of transferring strings, or visiting the onion address to copy the working address from the web browser.

Where this information could be helpful:

• https://workstation.securedrop.org/en/stable/admin/install/troubleshoot.html
• https://workstation.securedrop.org/en/app/admin/install/install.html