Update doc to new Tails Installer without splash screen (in Debian)

Originally created by @sajolida on #14677 (Redmine)

Feature Branch: doc/14677-no-splash-in-debian

Attachments

• Screenshot from 2018-02-17 14-00-50.png
• Screenshot from 2018-02-17 14-00-14.png
• Screenshot from 2018-02-17 14-01-07.png

Parent Task: #9005 (closed)

Related issues

• Related to #13301 (closed)
• Related to #15533 (closed)
• Blocked by #14650 (closed)
• Blocks #15411 (closed)