Clarify what Tails is and what makes it so awesome

Originally created by @sajolida on #9814 (Redmine)

Sponsor deadline: March 9

Blueprint: https://tails.boum.org/blueprint/explain_tails/

Subtasks

• #7492 (closed)
• #16536 (closed)
• #16537 (closed)
• #16538 (closed)
• #17248 (closed)

Related issues

• Related to #11778 (closed)
• Related to #12257 (closed)
• Related to #6863
• Related to #8948 (closed)
• Has duplicate #6161 (closed)
• Blocks #6857 (closed)