Document Tails Installer in Jessie backports

Start with reverting 4678906.

This shouldn’t block the first release.

Both graphical and command line.

Feature Branch: web/10953-installer-in-backports

Related issues

• Related to #10312 (closed)
• Blocked by #8805 (closed)
• Blocks #10734 (closed)

Original created by @sajolida on 10953 (Redmine)