Assistant: Clarify relationship between "OS" and Tails

Originally created by @sajolida on #9435 (Redmine)

During our tests we spotted:

• People who thought there was a version of Tails for Windows.
• People who were not sure which OS to choose because they had different computers at home.

In the second screen of the router we should try to clarify that:

• Tails is running independently from the OS installed on the computer.
• We’re talking here about the OS to install from and nothing else.

Feature Branch: web/assistant

Parent Task: #9445 (closed)