Document that Tails doesn't protect against BIOS/firmware attacks

BIOS is not dealt with by the OS -> it works as hardware -> if it’s compromized, Tails can’t protect you. Don’t leave your hardware unattended, and buy another computer (cheap, second-hand, linux-supported) if you have a doubt.

It could link to #6808

Feature Branch: doc/7076-plug_untrusted

Related issues

• Related to #8993 (closed)
• Related to #8845
• Related to #7076 (closed)
• Related to #6808

Original created by @BitingBird on 9116 (Redmine)